my c4 vette idles fine when started cold . as it gets warmer it becomes hard to keep it running , until it just stalls . it has all the symptoms of a faulty iac valve. i must continue to apply more and more throttle to prevent stalling. also there is backfire when i back off throttle . no code is set . any opinion will be appreciated . i have replaced tps, oxygen senser, map senser, checked for vacumn leaks-non found. egr working properly. what is it ??? please anyone , help ... thanks