Question for 450 owners

Hello I have a 1999 foreman 450 stock. My problem is that if I'm in a technical trail not very hard going but in first gear, the trail is a little rockey, my engine for no reason just shuts off like it missed a spark, or ran out of air or something then I,m stuck in that gear it's annoying. the bike does it also when forcing it a little bit in mud, I give alittle gas about 1/4 then let go of the gas, it happens when there is a little stress on the engine. I had a foreman 400 before with 26" vampires and it never stopped like that any information would be greatly appreciated. Thank you