Your problem is typical of a Temperature Compensation Board gone haywire -- we've all seen it. The TCB lies, and causes the gauge to erratically indicate high and low temperatures without rhyme or reason when it goes bad; and even when the board works properly, it dampens fluctuations so that you don't have overheating warning in advance. Bottom line, there is no good reason to have it.

I thought you wrote that you used the IPD kit to bypass it -- the kit consists of a single small wire with plugs on each end. So using the IPD kit means that you pulled out the board and threw it away, and replaced it with a single jumper wire across two pins in order to make the gauge work accurately (like the pre-85 gauges that never had the board).

That's why I suggested the IR thermometer in my other message. But apparently you've now revealed that you've kept the board installed, so forget buying the IR thermometer (although it is a useful tool, nevertheless)! The TCB is your problem. Get rid of it (and put in the jumper wire instead).