300E 1987 Overheating

I have a 1987 300E. Ever since I purchased the car it has it has an erradic over-heating problem.

When in slow traffic or at idle the car temp rises above 90. I can hear the auxilary fan cut in at above 90, but it seems too late and car has doesn't cool down unless I start moving again. I've never let it get to 120 as I usually am able to get up to speed forcing air over the radiator and cooling things down.

I had the car serviced last week. They replaced the thermostat with a 80 instead of an 87... seemed too low for me. When I left, the car got hot again immediately. Upon inspection, I found out the fuse was blown on the Aux fan....(stupic mechanic).

I had it served again this week by someone else. He replaced the thermostat again with an 87 degree one. He let the car idle for 2 hours with A/C on.. no overheating. Next morning in the commute to work, it began to overheat upto 110 degrees. Fuse is not blown, aux fan is coming on, but I just don't get why its happening.

Bad aux fan switch, bad fun clutch, bad temp gauge, possession?

Thanks in advance for any help.

Eli