What do I replace when hot air blows when set on a/c occassionaly?

Check the wire connector to the resistor.... Also hvac blend door operation.

Pushrod is exactly right. To elaborate a bit more, i've had the exact two problems on my truck, same year/model as yours. First, the blower motor speed issue: Supposedly, when the resistor gets old and weak it causes the wires and harness to overheat. I would actually smell mine when i ran my blower on high (when it actually worked) and there was physical signs of the insulation melting where the wires go into the connector. I had to jiggle and move the wires around just right every now and then for them to make connection for the blower to blow on high. Eventually i bought the Doorman 973-405 kit which is the resistor and wiring harness. I cut the old harness out and spliced in the new one and changed the resistor and problem solved. Also, someone before me had changed the harness before as there was butt connectors so i'm not sure if these things just have to be replaced every now and then or what. The new connector setup doesn't seem to run very hot. Since you already changed the resistor i think you can just order the Doorman 645-702 harness. Go check fitment at Doorman's website though since there's two harnesses depending on if you have single or crew/extended cab. For the AC randomly blowing hot...When this happens, if the passenger side is still blowing cold it is definitely your blend door actuator motor. This seems to be an epidemic with our gen trucks. You can spend $40-$60 and replace the part and be done. It's fairly easy to get to and replace. The root cause of the issue is that they packed so much grease for the plastic gears it eventually works its way into the little metal "sweeper" connectors that sense the actual location of the blend door. When it cannot sense the position of the door it "gets lost" and will blow hot or just do random things. You can open the motor up and remove the excess grease and clean the contacts really good with a q-tip and alcohol and that should work. I will say this though; I did this about 6-8 months ago and it worked like a charm until about a week ago and it started randomly blowing warm again until you turn the key off to reset it. I will take it out again and clean it and go from there but i'm pretty certain grease just worked its way onto the contacts again.