1991 240 running warm

[Zesty]

I've recently picked up a 1991 Volvo 240 5 speed sedan with 170k miles. The car seems well maintained, and I've driven it daily for about 3 weeks. My problem / concern is that the engine is running a little warm most of the time. It never climbs into the red over-heat zone, but hovers a bit above the horizontal position, maybe at about 60-70% of the gauge. I might be halucinating, but it seems I can lower the engine temp a bit by turning my cabin temp slider to the full cold position. To make things a bit more complicated, occasionally the temp guage drops down below horizontal and runs a bit cold. I have one more possible clue: I have no moderation in my cabin temp slider. It is cool when pushed all the way left, but any other position produces full hot air. I have checked the coolant level, and I am not losing any, the level is just between min and max marks.

I have a couple of thoughts:
1) Fouled up thermostat? (Which temp thermostat do you guys use?)
2) My 940 blew-out the coolant diverter valve in the engine bay (twice). Is the 240 suffering from a similar problem?
3) Is the cabin temp slider causing a problem with the circulation of engine coolant?

Any ideas would be appreciated!

[egads]

Temperature compensation board may be dead? What you describe sounds like normal variation.

[98T5]

When was the last time the thermostat was changed? if over a year, change it. even if less than 6 months. if YOU wasn't the person that changed it, I would do it again just to be sure. Thermostats are almost always the first place to look when a car is overheating. cheap, easy to change, worth it in the long run.