Why are my batteries getting hot?

[wevie]

My son has a BL Rustler. Using the MM5700. He runs Tenergy 7-cell NiMh packs. I can't say right off the top of my head what the gearing is, but when I put the MM in it, I used the recommended gearing from others running the same setup.

For months now, it has run great. Never gotten hot. Never thermalled. Now just this weekend, he has melted the shrink off of three battery packs in a row with the ESC being warmer than usual and the motor being very warm. . . I wouldn't say hot just yet.

Tranny feels as smooth as it ever was. Mesh hasn't changed.

What else should I be looking for?