Rough engine at 50MPH doing 1500RPM

I have a 1995 Grand Prix (3.1L with 96K miles) and when I'm driving at 50 MPH, with the engine turning 1500 RPM, there is a chugging sensation when I attempt to ever-so-slightly increase the gas, or when I'm using the cruise control and the car attempts to maintain 50 MPH without down-shifting. This symptom is more noticeable at 50 MPH, probably due to lower RPM, but it does the same at higher speeds.
Below 45 MPH, I can't detect a problem - only when the car is in overdrive. The only other symptom I've detected is when I put the transmission into Park and slowly try to increase the gas, the engine runs rough as it accelerates. I've taken the car to different garages and no one can diagnose the problem. They initially changed the plugs and ignition wires, but the problem remains. That same shop then identified the problem as a bad transmission. So I took the car to a transmission shop and they said the computer did not see any error codes, either from the engine or the transmission. The transmission shop claims that
without an error code from the transmission, they don't think the transmission is the source of the problem. They believe it to be engine related. There have been times when I've started the car, that it seems to be running on five cylinders. That symptom normally clears in a few seconds. But that has only happened twice in the last three months, once when it was raining (may be unrelated). To me it seems like the engine is stumbling at 50 MPH, as if the air/fuel mixture is incorrect.
Is it possible to have a bad transmission without an error code indication? Any ideas or help will be appreciated.

Thank You.

Since I received conflicting diagnostics, I ended up taking the car to a Pontiac dealer. They cleaned the injectors with their machine and the problem disappeared. In looking back, it's hard to believe some independent shops wanted to rebuild my transmission. The Rule of this story: It may cost a little more going to the dealer, but in the long run it is more cost effective and definitely better customer satisfaction.

What I was about to say. I have a Z34 that has the same problem. Few years back my dad diagnosed one of the OEM injectors to not close completly when was supposed to be closed and installed an injector from a Z24(A 2.8liters engine that was junked near by). I have not replaced this injector during the past 2 years until I got the oversized injectors and bigger fuel pump. As soon as I replaced the 6 injectors with 6 bigger ones the problem went away