Are the brakes locking up without you touching them? If they are, it possible that it's the thermal cut off (even though it might not seem that hot). Mine did that even after I went to a spektrum. It was doing it when I used a 7xl on 14 cells for racing, I ended up burning out two quarks. Now my third quark runs a 1515 2.5d (1700kv) on 5s lipo and I haven't had that problem again.
It's definitely not thermaling. Motor is about 135, controller about 145(at it's hottest point by the motor wires) and the batts. about 105 after a 5 min. qualifier. It occurs mainly when I'm really cookin' and start to slow the truck down quickly; not so much from slow or moderate speed stoping.