97 Lumina stumbles on acceleration

My 97 Lumina has a serious "stumbling" during acceleration. This usually begins once the engine has warmed up. The car always starts and idles perfectly (cold or warm start), but begins to bog and stumble during moderate acceleration and when at constant speed on the highway. Wide open throttle is great though. If I could always drive 90mph everything would be fine...

The car just had...
a full tune up
New Plugs
new plug wires
new pcv valve
new fuel filter
new catalytic converter

The chevy garage is at a loss as to where to go from here. Engine throws no codes. All readings are as they should be. They suggested maybe a faulty EGR valve, so I replaced it with a block off plate which made no difference at all.

Any other suggestions?

Have the shop check your car for vacuum leaks - cracked or worn vacuum hoses - also, our cars are notorious for intake gasket failure, which could also cause a vacuum leak. Does the stumbling only occur at moderate throttle application? Have them check your TPS.