Hey, thanks. I'm getting a lot of good advice, and I really appreciate it.

OK, I've replaced the monovalve already - that was one of the first things I did (without bothering to test it, naturally...). I'll try the test you recommended.

Now, what's up with the aux water pump? What should I be looking for, and if it's operating, how can I tell?

My biggest problem is that *none* of the things in the system seem as though they could be effected by speed.

When I'm idling, or driving below 45 (seriously - that's the magic speed), *everything* functions to spec. Set the heat to 22°C, put the fan on auto, and it'll keep the heat right there (verified with my digital thermometer). Want it warmer? No problem. Cooler? No trouble at all.

Above 45 MPH (I swear I'm not making this up), the heat coming out of the vent drops to ambient +10°F. (again, verified with my handy RadioShack indoor/outdoor digital thermometer)

So, I'm confused and frustrated at this point. I doubt there are many components left to replace, and the problem persists...