i work on an 81 300td that does the same needle jump on the temp. i think it's in the gauge but i can't have the car long enough to find out. that period car did not have harness issues, that's why i suspect the gauge. in the old days, i'd just pull 1 from aonther car and try it. not now, though.

duke 2.6 makes a good point about normal. being a kjet car, you want the highest normal temp for the highest control pressure. not enough temp and it will run a little richer than it could. really, that's true of any design fuel injection.