By 'cooling stack', I'm pretty sure he means the radiator. I was gonna suggest that as well. Radiators can get clogged with dirt and leaves, and sometimes even a plastic bag gets sucked up there, so check that area out and make sure nice and clean. I usually hang a bright light in where the fan is, and then look through the front to make sure all is clean. Hose it all off and blow out with compressed air (carefully, so as you don't bend the fragile cooling fins).
The fan clutch will turn easily by hand, with just a slight drag. My factory fan clutch was quiet as a mouse, and when my engine seemed to be running warmer than normal, I switched to aftermarket fan clutch, and now my truck sounds like a damn full-size commercial truck, it's way too noisy!!!
A/C may need a recharge, that will cure a lot of issues with it alternating between warm/cold.
And don't overlook the obvious, make sure coolant level is full, with clean coolant, and good thermostat. I installed slightly lower temp stat in mine. Of course, could be bad water pump too. It's pretty basic stuff, so you should be able to figure it out, and the repair won't cost you an arm & leg. These older GM trucks are worth fixing up, they're terrific trucks....
