91 Chevy Truck, 350 WITH THROTTLE BODY

Sounds like my old friend Mr. Sticking exaust valve is back at it again.
Do this, get an old tee-shirt-get the engine up to temperature (important)--drape the tee-shirt or an old rag in front of the exaust pipe--observe how it blows away from the exaust--have an assistant press the throttle just to get the rpms up to around 1100 or so while you still observe the tee-shirt--if at any time your the tee-shirt sucks into the exaust, even just a little, 99.999999123% of the time it is a sticking exaust valve to blame. Then tell your neighbors " I'm not nuts, I just want to dry off my old tee-shirt". LOL