I like to isolate the problem when this occurs, in a very un-scientific way.
When the gauge is high up next time, whack the top of the dashboard. You won't hurt it, just give it a good slap right over the temp gauge.
If the gauge moves, or resumes normal temps, then you know it's in the dashboard and you don't have to worry about actually overheating.
This could then be either ignored until there'd a good reason to take the dash apart, or the instrument cluster can be unscrewed, and the temp comp board replaced. It's all done with normal hand tools and the installation of the board to the cluster could be done on the kitchen table. Reassembly just requires a little patience.
If you're interested in doing this yourself, post back here, because it can be done for just the price of the little circuit board.
FCP Groton sells it as this:
1363117 Temp Compensator Board 86-93 $30.00 VDO
Their page is here:
http://www.fcpgroton.com/volvo240other.htm
Good luck!
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Rob Bareiss, New London CT ::: '87 244DL/M47- 225K, 88 744GLE- 209K, 91 244 183K. Also responsible for the care and feeding of: 88 745GLE, 229K, 88 244GL, 146K, 87 244DL, 235K, 88 245DL, 236K