my thought would also be overheating the ESC,.. however, I run a B40-L LMR on 10 zapped cells, with the 70 amp opto ESC,.. and a 13X13 carbon folder,.. which draws 84 amps. No problems with overheating in mine, even in a smart with no cooling provisions. I only have to run the motor 8 to 10 seconds though, as it's almost out of sight at that point

where did you purchase the motor and ESC? I'd contact them and tell them what's happening.