91 2.8L Problems

I recently bought a 91 S10 w/2.8L TBI 5 Speed V6 with about a 100K on the clock and now I have a problem I cannot find to fix and need your expert help.

It starts and runs fine but after warm up and not will always do it, it starts stalling at hiway speeds, clears up and runs good again for a bit then starts stalling again. I cannot predict when it will stall or run strong. When it stalls or loses power, it will stay running but at greatly reduced power. sometimes this also happens at lower speeds also.

It has the following new parts. Coil, dist cap, rotor, O2 sensor, thermostat, temp sensor, fuel filter, fuel pump, injectors, and a TBI rebuild kit installed. From what I can tell, it appears to be running in closed loop when warm. FYI, computer codes are always 12's and nothing else.

I have been told that a faulty or clogged EGR valve and or line could cause these problems but have never had one go bad on me so I could verify that to be accurate. Is this a possibility or should we look elsewhere?

Any help on finding this gremlin would be deeply appreciated by both me (financially) and my step son who needs a truck!