It sounds like either a tuning issue or a heat issue. Since you're mudding, a heat issue is most likely.

When it stalls, how much mud has caked itself into the cooling fins on the engine?



While it's never safe to assume, one can deduce that, because he's A: in the mud, and B: Complains that it cuts out when he applies full throttle in the mud, that he has a nitro monster truck, stadium truck or buggy of some sort. Most likely a monster truck.