1990 Sierra stalls when warmed up

I have a 1990 GMC Sierra with the 5.7 engine, it seems to run strong but I get an occasional miss when driving after its warmed up. I put in fresh plugs and no chage. the worst of it is that sitting at a stop light with the truck in gear the truck stalls while idling unless I pu it in Neutural. any ideas why this happens?

The IAC valve may need cleaning or replacing or the throttle body may need cleaning.

The engine temp sensor may be sending the computer the wrong signal when the engine is up to operating temp, thus giving the engine the wrong fuel/air mix.