2000 Tahoe issues

Hey everyone, I was wondering if anyone could give me some insight on a problem I have been having with my 2000 Tahoe...3.2 liter v8...basically the idle is considerably lower than it should be(between 350-400rpm) and it dies while in low idle or if I am at a stop light or in stop and go traffic and have the ac or heater on it will die, and on the advice of a local mechanic I have changed the throttle position sensor, sparkplugs, oil and serpintine belt and am still seeing these issues. I have had a diagnostic ran on the truck and it isnt producing any codes and I am not seeing the check engine light coming on....I dont know where to go from here and am at my wits end Any suggestions would be helpful...Thanks for reading!