My 300 SD was running at a constant 100 c. I took the thermostat out and tested it which seemed to be opening fully. Ran it w/o thermostat and seemed to run too cool not reacning 80c which Im told is s.o.t. So I drilled 3 holes in the outer ring of tstat to get more flow thru radiator and at 1st seemed fine never getting above 80c but on a long road trip after 2hrs. highway speeds my temp gauge started to rise above 100 c and then erattically bounce around 100 and above but never in the red zone. I will change the stat out with a 160 one when I get home but wondering why my temp. Gauge was fluttering. Any one else have a problem to what I described. I hope my problem is in thermost as trying to avoid pulling radiator and having it flushed and cleaned. I got the idea for drilling the holes from a diesel giant writeup he uses one with 2 holes in winter and 5 holes in summer I went in the middle with 3 holes as a test I might go to 5 holes but know it will give me longer warmup times and then be too cool for winter.