I have a yamaha xt 125r 06 model and my spark plug is constantly sooted and i cant go no faster than 45 mph as it loses power and sometimes cuts out

Thanks for contacting FixYa. Fixya is going through a software upgrade so you will get a lot of responses to your issue. We all have the same goal in common - to help you the valued customer. I'll lay out a strategy that will get your power problem resolved. Other techs may comment to advance the solution.
http://www.bikez.com/motorcycles/yamaha_xt_125_r_2006.php
Free Service Manual:http://bmanual.com/Yamaha-TTR125-2006-Service-Manual.html

Root Problem:Your bike is running too rich from over fueling! Won't build torque or rpm.
Steps:

1. Clean your air cleaner and spray it with light filter oil.
   
2. Check your exhaust pipe make sure it's not plugged or restricted
   
3. Gear ratio - too many teeth on primary sprocket?
4. Install a new or clean spark plug for color testing. Want a cocoa brown.
5. Jetting - Needle Valve - Raise the cir-clip a notch at a time lowering the needle into the pilot jet. Mostly effects the mid-range 35 - 60 mph.
6. Jetting - Main Jet (should be OK) if it has been changed/increased to a larger orifice you may want to go back to stock. This usually effects speeds when the wire is tight and your running is WFO. Wound Flat Out.