3.4l automatic overdrive

I have a 1994 Camaro with a 3.4l automatic transmission with 230,000+ miles on it. Recently, in overdrive, the car dances and shifts in and out of gear, especially at speeds 60+ and passing. It doesn't do it in drive, only overdrive.
I had the transmission fluid and filter changed after I noticed it, but to no avail.
:shakehead

Does your service engine soon light come on at all when this happens?

as a matter of fact, it does! But I thought that was from my EGR valve because it needs replacing (just had it checked. codes 75 & 76, I think, came up)

Well if it is doing off and on during this time, and only during this time, then it is most likely a misfire rather than the transmission. Normally the EGR sets off the light and it stays on for a good period of time, mine does the same thing, I typically just ignore it since it's nothing major. If the light is coming on and off, on and off.. and repeats that, then you definitely have a misfire.

The light does come on, stays on, then goes out. But it doesn't flicker. It would be terrific if it were only the EGR valve! $$$$$$$

Well thats what I meant. Like does it momentarily come on, then back off... and momentarily back on again. Not in a flicker kind of fashion, but more of a momentarily comes on and back off and a few seconds later comes back on again.

yes! That's It!!!!

Yeah, you have definitely got a misfire then. I just recently had the same thing happen. Turned out to be my coolant temperature sensor that was causing it. Have you recently worked on or around the thermostat/throttle body area? Because the sensor is right next to the thermostat and they break really easily. Also check your spark plug wire configurations and make sure they are all correct.

I did have my oil changed recently. Could they have accidently jolted it or is it even in the area of the sensor?

no, the oil drain plug is no where near it. Only thing I could recommend is getting your codes scanned and hope something comes up that is relevant to your misfire. Try going to Advance auto parts, and have them scan it. They'll tell you that they're not able to since yours is a 94, but let them know that you have an OBD2 port and to put in the information of your car (year, make, model, engine) into the scanner and then read the codes. Because when they hook the scanner up directly without putting in the information first, it will try to scan for OBD2 codes, when you have OBD1. I would try to help you some more, but narrowing down whats causing a misfire is like searching for a needle in a haystack. But try to start with the basics like changing the plugs, wires, making sure the ignition coils are working properly. Maybe this website can help a little, it says for OBD2 systems, but all the same stuff applies:
http://www.aa1car.com/random-misfire/

It does say that a faulty EGR could be the cause of a misfire, I never even knew that, so maybe that is something worth looking into since you said you've been having problems with yours.