Try changing the thermostat, its a cheap part and it makes a huge different, I had the same problem on my 92 sc400. Cheers.
:o
Mine too man! I was thinking about starting a thread on it as well.
Last year it seemed OK, but this year is cold as hell in Iowa and I have to do all kinds of stuff to get it warm.
First, I crank it up, let it warm up some. Secondly, I jump in and hold the throttle up to ~2500-3000 for about 20-30 secs to get it up to a reasonable temp. Next, while driving, I floor it every chance I get to get it to warm up. And finally, I put it in Neutral at stop lights and hold the throttle open as I mentioned before. This all actually works reasonably well, but my gas mileage sucks because of it (understandable). Unfortunately, as soon as I slow down, as you mentioned, it starts cooling down. I've tried all vent settings and nothing holds the temp for any extended amount of time.
The final clincher is that the engine temp is just below halfway (normal operating zone) and stays there regardless of what the temp blowing out of the vents feels like.
I do have to say once it gets REALLy warmed up, its too hot, but it's taking FOREVER to get there. Maybe someone out here knows what we can do.
yeah man when it gets warm it gets REAL warm.. only difference is my engine temp is just a hair above normal operating zone then it will drop down to regular and stay there... let me know if you ever figure it out and i'll do the same if i do.