I have a 90 S10 2. 5 and 5 sp. 175K, engine stumbles at cruise speeds with light throttle or when backing off throttle. No codes and have changed all sensors twice, ecm, injector, fuel filter, fuel pressure regulator, fuel pump, plugs and wires, dist cap and rotor, pickup coil, no help. Scanner shows everything working right. Checked timing. Truck will accelerate and run fine, passes emission test every year. It just bucks and kicks back at light or no throttle about 45 to 55 mph. When it starts stumbling I can step on the throttle and it stops and accelerates fine????? Thanks for any HELP!!