Ok, so I think I might have the problem nailed, but I'm open for suggestions and other ideas. Here's the sit.:

Last night and this morning, I got in my 92 and started driving, and as usual, kicked the heater on. It gets up to operating temp (around 195-205) and everything is fine. Then all of a sudden, it blows cold air (usually at a complete stop). If I shift it into N, it still blows cold, even with revvig the engine, but as soon as I take off in gear, it blow hot again. During this stop, I also notice that the gauge reads a bit over 210, then I hear the fan kick on and everything's kosher again.

I think I need to burp the system and top off the coolant.

Any other ideas?

 

Are you loosing your vacuum when idling? This would cause the heater unit air doors to flip over to the cold position. When you get up some vacuum again when driving, the door flips back to heat.

 

it'd be a safe bet to burp it and see wut happens