I have a 99 chevy k1500 z71 and when i am driving at least 30 mph and 3rd/4th gear with a steady hold on the gas pedal (RPM range 1500-2000) the truck surges, you can hear the engine speed increase slightly and it will everything feels rough then kick it self out and completely fine. this problems happens every 10-15 seconds while driving at a steady speed on the road or highway. have checked tranny fluid, plugs, and wires.