Chattering volcano
 

I have a redcat racing Volcano epx that has been upgraded to a brushless system. When running on pavement there are no problems but when placed on grass, the runs for a short time then the movement shuts down and chatters. We turn the esc off then back on and same thing happens.
The truck has a hobbywing 10bl60 esc, a 3650 4 pole 3900kv motor, steel spur gear.
Does anybody have any ideas of what wrong and any recommendations on fixing this?