Originally Posted by Ghostvette

In a nutshell, this is the basic coolant path through any engine; Radiator, lower hose, water pump, heater core/engine block, upper hose, radiator. Coolant circulates through the entire system, otherwise air would get trapped and cause coolant temperature to rise. The temperature you feel inside the car is dependant on the position of the blend door. It will block off hot air from the heater core and allow cool air from either the AC or the outside to enter the cabin. If it's not closing properly, it might explain why sometimes you get hot air and sometimes you get cool air.

Some things to look at:
1) Have you ever replaced your cabin air filter? Some of the early Z's didn't have one, so anything that dropped into the space between the hood and the firewall could find it's way down into the HVAC system. Fresh air is brought into the system through vents that are in the cowl area of the car. If you have never changed yours, it is possible that it is so full of debris that it has allowed some of that debris to enter the system, blocking the operation of the blend door.

2) Vents in the cowl area need to be cleaned. If you park the car outside, leaves and other debris will clog these vents and eventually break down, ending up as dirt and mud inside the vent system.

The cabin air filter is behind the glove box (there are a couple of threads on the forum. Search 'cabin air filter' or use the link to the FSM in my sig). Once the filter is out, you can use a shop vac with an attachment to clean dryer lint (it's a 1.5" hose with a crevice tool on the end. It usually has an adapter for shop vac hoses) to stick into the area below the cabin filter to suck out anything that might be in there. It's flexible enough to possibly reach almost to the center of the dash to pull out any debris that might be there. Good luck.