A gate valve (cheap) or a needle valve (not cheap) will do better for flow modulation. Balls are hard to control, they want go all out or do nothing at all.
What kind of board is the 2500 in? Does it read the internal diode? If not, maybe a minor mod would allow it to. Multiple temperatures reading (thermocouple, diode, socket thermistor) would allow you to spot oddness easier. And yeah, I think your Duron is from before AMD started putting thermal diodes in all their chips.
T thermocouple are a bit more accurate than the K flavor. More expensive too. Just a matter what you consider worth it. I didn't check the specs for the meter you linked to, tinyurl is blocked at the firewall (too many bad people disguise their links with it).