Helm has resistance specs, as temps go up resistance drops. I don't think its a real practical test unless you have an accurate reading of the coolant temps. also that sensor is not exactly the most accessable.

none the less, here's what it lists:

degrees F / K ohms
14 / 7-12
50 / 3-5
68 / 2-3
122 / 0.7-1
176 / 0.2-0.4

this seems like one of those situations that a # of things could contribute to your symptoms. I had similar issue that I never sorted out, and with my motor half apart I won't figure it out anytime soon. <img src="/forums/images/graemlins/rolleyes.gif" alt="" />

off the top of my head, will idle speed kick up if you switch on AC/heater?