96 Lumina Stumped Techs

Hi I Own A 1996 Lumina With 77 000 Kms
Never Had Any Troulbles With It I Have Owned It For 3years And It Only Had 55 000kms The Problem Im Having Is That When The Engine Is Warm And In Closed Loop While Driving Down The Road At About 2000 -2300 Rpm Its Lacking Power It Feels Like It Shifting From 1st To 3rd Gear I Took It To The Shop And They Got A Second And Third Opinion From Two Tranny Shops In Town And They Both Say Its Not In The Tranny I Have Replaced The Plug ,ignition Wires And Fuel Filter And Clean Out The Throttle Plate Thinking Thats Whats I Thought Was Wrong The Repair Shop Did A Fuel Injector Clean And Did Help Slightly But The Problem Is Still Their The Tech Is Stumpped Any Thoughts Would Be Appreciated Thanks In Advance Hope Someone Out Their Has An Answer

Maybe it's the vaccuum modulator, if it is worn out the shifts feel strange. If it is not in the tranny it should be something in the engine like air filter or maybe a vacuum leak which hurts acceleration a lot.

I have the same problem with my 1997 lumina. I have changed the plugs and wires. Had two fuel injector flushes. The dealer calls it fish bitting and blaims the gas that I am putting into it. I don't buy the gas issue because I have no problems with the same gas in my chev truck or volvo. It has been going on for two years now and I keep thinking that I should just dump the car. Let me know if you have found a solution.