2002 Isuzu Rodeo Question AC Blows hot while idiling and col

I have a 02 Isuzu Rodeo and while im sitting idiling my ac blows hot but when im driving it gets cold ! I mean it I will be driving and it gets cold but will stop at a red light and it get hot or while letting someone run into the store it'll get hot ! Oh and i bought a gage thing from the parts store and when we checked it it was in the yellow part which is warning ! Please help! I plan on taking it to the shop sometime this week but trying to see if anyone knows what this could be !