I was messing around with my power stage this morning, tracking down a weird start, and afterwards the temp gauge was running at 10 o'clock, rather than the normal 9 o'clock. One quick slap on the dash and the gauge went back down. So does it affect the temp gauge and temp board comp?

Can't imagine the power stage would affect the temp gauge.
Especially assuming yours is a '91 as in your sig - this is past the era of totally rotten wire insulation that affected so many 240s up till mid-'87.

I'd almost bet real money that your trouble is from the temp compensator board.
Go to IPD's site: ipdusa.com, Volvo section, 200 series.
Go to cooling system, find the Temperature Compensator Board Bypass Kit.
See the .pdf file for instructions for the kit.

That's your how-to.
Solder in a wire where the jumper would go.
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DAMHIK: Don't Ask Me How I Know - - - Sven: '89 245, IPD sways, electric rad. fan conversion, e-codes, 28+ mpg - auto tranny. 500 mi/week commute. '89 245 #2 (wifemobile). '90 244 (spare, runs).