Picked up this JC Penny special Wabash 8587 GP9 "Fundimension era" recently and it was running good in reverse and very erratic in forward. I thought lubrication at first and did the usual lube and oil, but problem still existed. I investigated further by popping the armature cover and cleaning armature and brushes. Reinstalling the brushes I found the spring on one brush was sticking to the inside of brush holder the spring was too long. Trimmed the spring back and that solved my problem runs like new now.
