[W126 Coupe] Running warm

[Pavel]

Hi

Have a new problem with my 380 sec.  Temperature gauge is acting weird. 
Keeps on running at 90 on freeway, than climbs past 120 than instantly
drops to 80.  Most of the time sits around 90.  Just put a new thermostat
in, still same story.  Changed the temp gauge no difference, changed the
temp sensor no difference.   Sometimes the temperature guage will stay
right at 80 for 20 minutes and then will start acting strange. Expansion
Tank seems to be under pressure.  If the gauge is showing 110 deg and I
shut the car off and right away put it in the on position it the temp gauge
displays 80.  As I shut the car off the gauge will peg out while the engine
is stopping. If I accelerate hard on the freeway it will drop the
temperature temp to 80-85 range. When I coast on a freeway do get back to
speed limit it will go back to 90-100 range.  If I turn the heat on it will
bring the temperature up I'm very confused.  Should I try a new radiator? 
Also I'm in minnesota and it is about 50 deg right now, car should run just
a hair above 80, like my 560 is.

Also noticed a coolant leak in parking lot after changing thermostat
thought it was came from the thermo housing when got home no leak
everything is dry with plenty of coolant left?
Pavel