8XL skipping

I have a problem with an E-Maxx. It was previously running well (35 mpg), now it is skipping – like cogging – with full throttle. It began as going very slowly with full throttle (like dead battery), so I increased the throttle EPA which increased the speed, but the skipping began. The motor does not get hot after 3-4 mins but the controller gets to 120F and capacitors 150, batteries and motor are nml for temp. Set up – 8XL, 12020, 12 cell IB 3800, 18T and Strobe Slipper. I removed the EBEC and went to a battery pack, no change. Ant thoughts on what it could be? Many thanks from a rookie!