Here are some tips that should help alot:
If it is your motor, give it a re skim and seat the brushes in again (I think somebody already said this)
Have you glued the edges of the chassis and top deck? if not do this.
Move the PT as far away from everything as possible.
I had glitches with my Pro4, I mounted the receiver on its side and on top of the servo.. all gone
My freind also had the same results by doing this with his Pro4.
Does it glitch with all crystals/frequencies?
Are any of your speedo/servo wires spilt? and keep them tidy and away from each other.
If you are using a fet lead keep it as far away from everything else as possible.
Have you tried different sets of cells? check too see if one set needs re heatsrinking.
Also, if you are going too put your own capacitors on the motor, take the circuit board off it first, adding new ones will half the affect if you dont!!
Interfeirence is a pain when you dont have a clue whats causing it . Good luck!