There are way too many variables to be able to help you out on this. I can give you a few idea's but several system's need checked to even give a good guess. I'll assume you have a 2.8L engine but most of these will go for the 4.3L and the 4 cylinder also that your check engine light is not on. Should it be that's is the place for you to start.
Since your just know questioning what the problem could be, I'm guessing it has gotten worse with hot weather. That would point to a fuel system problem and lean mixture in this case testing fuel pressure both output and deadhead pressure's. Once you have these numbers it will give you a direction to start your testing. More than likely a lean mixture caused by weak fuel pump, restricted fuel filter or fuel pressure regulator spring broken. If these test out take a look at the spark system for poor coil output or a problem with the elect. spark control system. Good Luck and before spending to much on parts your not sure are the problem most all Repairpal shop's have the equipment to test out the system's and make the repairs the first time without a lot of wasted money throwing parts at it.
