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Junior:
Thank you very much for the idea! As it turns out, I had already replaced all the plastic "ducting" in front of the radiator when I rebuilt the car, but I checked again anyway. Unfortunately, everything was fine. On the "bright side" of things, however, I did find the correction to the possible "design flaw"!
Since I have now seen this "issue" in three B230F powered 240s, I had to believe that it was a "fundamental" design problem. As I stated in my initial post, opening and closing the heater control for a few seconds let the engine cool back down into the "normal... dead center of the gauge" zone. Based on this, I made the assumption that the coolant was boiling in the head (at the temp sensor) which was causing the gauge to read a very high temperature.
I finally spent the "big bucks" and installed CLEAR high temperature hoses to the heater core. Once the "overheat" condition appeared, I cracked the heater open and watched the fluid flow through the hoses... end result... a TON of air bubbles BEFORE fluid returned! CONCLUSION: Either A: fluid was boiling, or B: water pump was cavitating and the resultant "air" was being trapped in the head (Physics doesn't support this one very well at low RPMs).
As it turns out, when the heater is turned off, the valve BLOCKS flow out of the head and back into the pump, quite unlike "American" heater valves which don't block, but rather redirect flow around / past the heater core. As a result, I installed a semi-kludge-custom bypass valve at the firewall for the heater core and then tested the impact.
I drove the car in 94 degree weather for 20 minutes, pulled back into the driveway and let the bumper TOUCH the garage door, and then let it sit there for 30 minutes idling. The end result was that it NEVER overheated as indicated by the temp gauge or the thermistors in the upper and lower radiator hoses!!!
Once I get "done" refining the (currently crude) bypass valve for the heater core and its controls, I will make the details available on the brickboard.
Once again, Thanks for your input, and I hope that you and others find the above information useful! After all, a Volvo saved the life of my wife and our Boxer dog... its the least I can do! If anyone is interested in what was "left" of the car after the accident, let me know and I'll provide JPGs!
Later,
John
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