check your drive potentiometer

Yes it could be brushes. If you do replace them check to see if they are uneven, if so a comm atone may he needed to smooth out the armature. I would also check the forward and reverse contractor tips for any pitting sticking or burning. I have this issue with the large number of RC's at my customers account. Verify the fed/rev switches are good do a volt drop test to determine this. I would say the issue is in the switches being faulty or may just need to be adjusted. Good luck!