motor shuddering and fallen on face

89 quad 4 with five manual. hard to explain what its doing. lets just say motor runs fine at low rpm's, but when u go to get into it, it kicks back at u like a bucking feeling. motor cuts out and rpms drop. but if the tires are spinning like in mud or snow it doesnt do it. local mechanic has no idea. tried new crank sensor, two different sets of coils, housings and ignition modules. also replaced fuel filter. still no luck. next step ecm or fuel pump. i have no ideas. any sugestions???

Never mind it was the fuel pump....