I'm using the Yamaha P-85 Digital piano. Unfortunately it has lately had some problems. When I'm playing to notes next to each other (especially e and f (mi & fa) or b flat and b (si & si sostenio) by turns fast the keyboard is playing just the note I'm touching first. For example if I play fast e, f, e, f, e, f, e - I can hear only the first e and that's it..! Sometimes this problem just disappear but sometimes it comes back with another problem. In that case when I play just one key the keyboard is actually playing many keys. So basicly in the first case when I'm playing a melody I can't hear all the notes I'm playing if they're close to each other. In the second case the playing will sound really off-tune, because the keyboard is adding some notes I'm not actually playing... Is there anything to do for this problem?

What you're beng told is to reset the instrument. T/his is als refered to as reboot, or reinitalize. The instructions are at this link-

Resetting Yamaha NP31 Portable Keyboard

Either a faulty power supply or more likely a damaged socket

Reset Instructions:
1) Try a partial reset by holding down the last white key while turning the unit ON.
2) If you haven't recorded any performance(s), for a full reset, hold down on the last white key AND THE lst three black keys while turning the unit ON.

Haha yes - I know the feeling...fingers are nowhere near the keyboard the "BANG!" a note or a cluster of notes happens out of nowhere! :)

Easiest solution will be to take it in to our local Yamaha Dealer and have them clean the contacts. Dust and hair can get between the keys and cause all sorts of problems with the sensors/contacts.

I have learnt to clean them myself using a photographic blower brush, and if you're daring, give it a go. If not - don't risk it :)

First try looking in your transpose settings. Refer to your manual to see if any arpeggiation/unnatural chord settings were inadvertently created by you or made by another user. If you believe it is a software problem you can not solve, back up your memory bank, remove your memory card, and hard reset your keyboard. Insert your memory card and check to see if your problem still exists.
If it is a technical problem, it sounds like you may need to get it checked out by an engineer/repairman. Sometimes when a digital keboard's keys collect too much dust/debris they can trigger multiple keys that aren't being pressed. Think about it like a key getting caught on an upright piano and pulling back the hammer next to the one that was played. A digital piano works off of one or several printed circuit boards or PCBs. The printed circuits usually can not short circuit because the wires and circuits sit in fixed positions. If an alien conductive fiber or series of conductive fibers connects a conducter to another, or if there is any humidity inside your piano it may cause circuits to misfire when a circuit is connected in the area. This could explain why notes you are not playing are being activated in the area in which you are playing.

If you are in a humid area and you suspect that this may be the cause you can attempt to draw out the humidity before getting a an expensive diagnostic done. Find a box large enough to house your keyboard. [If it is on a stand take it off and] place it inside the box. Cover the keyboard with plastic wrap (so as to prevent any foreign objects from getting into the instrument) and very carefully surround the piano with white rice. Remove the plastic wrap, close the box and leave it alone for a few days. The rice draws out any moisture from the electronic components. Very carefully (again so as to avoid getting any rice inside it) remove the keyboard from the box; remove any dust from the body, face, keys, and other components. Try your keyboard now. If humidity/moisture on the PCBs was the problem it should be solved. If not it is time to take your piano for a tune-up.
Hope this helps!

Have a professional tune it for you. It may be that some strings have loosened/tightened.

Check your manual. You should have an option to enable and disable the touch sensitivity.

First thing is to check the connector is fully seated... sometimes most of the contacts make but one might not. Beyond that, contacts in the pedal are next suspect.

All sound is generated digitally and the pedal doesn't modify the sound, it just sustains the generator. This is a strange problem and best suggestion iis to find another unit and compare... it may be a characteristic of the design. There isn't anything that you can do to change this. It could be there is a bad ground inside the unit or the digital data is corrupt in the processor. The fact that it is on a particular set of notes is strange...I just checked on my Yamaha piano that is essentially the same electronics and I notice that on one octave above the middle when one strikes F# and G they "ring out". Now this MIGHT be due to the characteristics of the speakers. Those particular notes seem to clash more. I don't have other speakers on mine and unfortunately you did not mention what brand model you have attached. Consumer grade speakers like fancy stereo speakers do NOT do well on these instruments. One has touse pro grade speakers AND use proper interconnecting cables. The speakers, powered type, must be powered from the SAME receptacle as the piano to avoid ground loops.

You will likely have to double that budget to get anything decent... but look for a Yamaha in the PSR series (61 keys minimum). To get a full 88 key, you might have to be up in the $400 range for anything really decent.