Most thermostat problems that i've herd of with the 40V 4.2 involve one that's stuck closed. My temp doesn't go past the halfway point but it does vary quite a bit from normal. Is this what happens before the thing fails?
You may have a thermostat that works hard, i.e. takes a lot of time to open / close. When the engine gets hot, it slowly opens, but then it takes so long to close, that the engine cools too much, and so on. This whole process stabilizes at a lower temperature than it should. Whether it's this or not, you should replace the thermostat, it's inexpensive.
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