I was thinking two things (if I try for three, my brainpan overheats).

The behavior of the temp guage with the compensator in place an working is that it will seem to be fine until it's just about to overheat, then the needle will "spontaneously" and "sporadically" rocket up to the top. It will seem like the car suddenly overheats with no warning. Really, it has been happening gradually, bt Volvo didn't want you to stress out over it until it was definitely a problem. By the way, it will be sporadic due to changes in ambient temp and driving circumstances: the faster you drive, the more airflow through the radiator, the cooler the coolant. And you said they were forecasting days of rain, so I'm going to assume you're somewhere in the area of New England, where the ambient temps have been all over the road.

Now the first thing that actually sprung to mind was that the water pump was seized, and the squeal was the belt rubbing on the immobile pully. It occurs to me, though, that that would be less describable as a "squeal" than as a "banshee-like screech," rendering the car undrivable, and the car would overheat quite dependably, rather than sporadically, anyway. So, I'm going to dispense with this idea now.

So, basically, other than my hypothesis that the temp compensator board is okay (for what it's worth), you can all just ignore me now. I'm going to bed.

-EdM.
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'90 240DL Wagon 'Lola' -- '72 1800ES 'Galadriel'