The belt on my Chamberlain Whisper Drive dipping

Make sure the dip switches on the transmitter match those of the receiver

Find your manual here:

http://www.chamberlain.com/
It is very simple DIY with the manual.

could be that the remote operation or cycle operation needs reprogramming as it indicates that the program has a corruption in it and the action is lost by that corruption
follow the user manual and do the reprogram sequence

Have you read the manual, you can find it here Chamberlain Garage Opener Manuals

You can find the user manual here Chamberlain Whisper Drive

You will need to research your particular model--there's probably a model number listed on the opener.
here is a link to the various owner manuals available on line:

http://chamberlain.custhelp.com/#s=eyJzZiI6eyIxNzYiOnsiZmlsdGVycyI6eyJub190cnVuY2F0ZSI6MCwicGFnZSI6MSwic2VhcmNoVHlwZSI6eyJmaWx0ZXJzIjp7ImRhdGEiOjV9fSwia2V5d29yZCI6eyJmaWx0ZXJzIjp7ImRhdGEiOiJsb29zZSBkcml2ZSBiZWx0In19LCJwIjp7ImZpbHRlcnMiOnsiZGF0YSI6eyIwIjpbXX19fSwiYyI6eyJmaWx0ZXJzIjp7ImRhdGEiOnsiMCI6W119fX0sInNvcnRfYXJncyI6eyJmaWx0ZXJzIjp7ImRhdGEiOnsiY29sX2lkIjotMSwic29ydF9kaXJlY3Rpb24iOjF9fX19fX0sImMiOjJ9

There is a button on the side of the wall-mounted remote. It has a pic of light bulb next to it. Press it once - the motion sensor is on and the lights will be on once it senses the motion; press it again and the motion sensor will be off.

I just had that problem this morning and the cause was that the springs that help hold the weight of the garage door were broken. This caused the garage door opener to raise the garage door a fe inches and then drop again. We called in a professional and $200 later we had new springs and everything works fine now.

You could always use "Belt Ease" from an Auto Parts store. It's made for squeeky belts.
-Lonnie