This just started happening yesterday, 1997 Rodeo. I searched the forums and didn't find anything.

At idle, my heater blows cold air, and while driving I get hot air. I fiddled with the temp switch and it seems okay - does this directly actuate the heater core valve or is it vacuum controlled? I used 4WD for the first time in a while last night, so I thought maybe I cracked the diaphragm on the 4WD actuator or something, but it doesn't seem like the valve is vacuum actuated.

This temp change occurs very rapidly, so it doesn't seem like a cooling flow or thermostat issue, but anything is possible I suppose. This thing has 151,000 miles on it, I haven't touched the cooling since I got it at 123,000 miles.