89 Jimmy 4.3L Vortec stumbling on acceleration

Starts right up and runs good for 5-6 miles then misses,stumbles when accelerating. Will accelerate if slowly giving it gas. Park for 1-2 hours and it will run like new. Happens every time. Cleaned LEGR twice and installed kleen screen, new plug wires, PCV valve, fuel filter, injector cleaner in gas tank twice. No help. No check engine light or codes stored (using jumper wire ). It will accelerate and run good after it sits. Thanks for any help or advice.

Make sure both injectors are spraying,when this happens, take off air cleaner and look, if ok do a fuel pressure test.

Sounds like the problem starts about the time that the system goes in to closed loop. Check for a bad oxygen sensor. The system ignores oxygen sensor output during open loop operation.

If that checks out ok, check the IAT (inlet air temperature) sensor voltage and compare it to the ECT (engine coolant temperature) sensor. Those two sensors should read the same from stone cold through operating temperature. IAT may read a few degrees colder with the engine running at operating temperature, (IAT is passive, ECT is active), however if you shut off the engine and let temps stabilize for a few minutes, they should be close.

Since posting the problem, made 8 trips (15 miles or less) and it stumbled once for about a second and cleared up. It's been running great ever since. Perhaps the kneen screen on the LEGR or the injector cleaner is clearing it up. Time will tell. Will keep your advice in mind. Many thanks for your help and taking the time to do it!