Ford Repair: Idle Problems, ford ranger, idle control valve

Question
I have a 99 Ford Ranger XLT, 4 cylinder, automatic transmission. Recently I had to get my alternator replaced because the battery light was on. My dad is a car dealer and we fixed that problem! I thought all was fine, but now I have trouble with my truck idling. Sometimes when the truck has been sitting I try and start it up and it sometimes requires me to press the gas and "revv" it up  to a certain RPM's and then it idles fine. Other times I can go out and start it up with no problem. Also there are times when I think it's doing ok, then when I stop at a light or stop sign with my AC on it acts as though it can't idle and idles really low almost going dead. My Dad and I are trying to figure out what the problem is! He believes it is bad gas or a spark plug not firing. Is he right?

Answer
Is he right? Don't know without some preliminary testing. Checking for codes would help, even if the check engine light has not come on. Could be as simple as a sticking idle control valve, or as serious as a leaking injector.

Check for codes first.