8XL running slow

I am having a problem with an E-Max running way slow. It’s running like the last minute before the batteries die. Previously it was doing about 35 mph for the last 5-6 months. 12020/8XL/12 cells. Temp has been fine on an 18T and strobe slipper. Batteries are fine – I discharged then recharged. I put it together with stock Titans and slipper and it runs fine. The drive train is intact – steel idlers, MIP CVD/Center drives. The 8XL spins smoothly and no apparent damage. My only thought goes to the controller. It did over heat when I was 1st working with the gearing, but has run fine ever since. Any recommendations?