This thread has moved off from the batt capacity some what on to transistor theory!
I think the reason that the H1357's blow is because they have the PWM signal, and I could go very technical on you and explain why this could stress devices more. But I won't unless some one asks. As for a replacement I think I saw some one on a German site stuck some higher speck transistors on a bit of strip board, but for the life of me I can't find it again. I recon that adding a hand full of diodes could improve the reliability loads, along with putting a bit of aluminum plate on the outside of hull bottom coupled to the internal heatsink. By the way, the 'standard' motor requires around 9A under stall conditions with a healthy 7.2v. This could be why some failure is happening with fresh 3800mAh, and not with older 1700mAh batteries. Fresh 3800mAh will give 9A with out much volts sag, old 1700 at 9A would sag.

Any way that's my worth in this discussion.

Yours Simon M.