E-Maxx battery ?
 

My set up was 12020/7xl with 54 tooth ofna spur on 14 cells in an old E-Maxx. I have been having run issues, the motor is very jumpy, not smooth at all. I am pretty certian it is the batteries. The controller has been checked by BK and the motor is new. Connections look good. I have noticed the cells have been dying. I have IB 3800 and 4200. The 4200 are particullarly getting fried/over heated. I have geared down the pinion to 14 tooth. I have put the Titans back on with stock spur, and some of the batteries are still dying. Is it common for the brushless to draw to many amps and kill the NiHi? Is Lipo the only answer?

If they have been getting "overheated"... then the amp draw is stressing the batteries.

Good fresh 4200s should do pretty good, but it's not uncommon... for nimh to struggle, especially if they are not in great shape.

Lipo or A123 is the best answer imo.... Dewalt pack off ebay (built into a 5s2p 4600mah) or a TrueRC 8000 pack both cost about the same as getting a pair of 7 cell 4200 packs and I think you will be much better off with either one.

Checks.
 

have you checked to see that your drivetrain turns smoothly without any resistance? It sounds like you are stressing the electronics due to to much resistance in your tranny, diffs, or wheels.

Jerry, I just went in, replaced the diffs, lubed them and checked the MIP CVDs, so I think the drivetrain is ok. It seems to be doing well with the titans.