my '85 300D Turbo is also the same way, if its between 80 -90 deg F outside temperature, my water temp gauge reads between 80 - 100 deg.C if i'm cruising between 70 - 75 MPH with the AC on and no load but just myself, my question is why is it that if i drive uphill or drive at about 85 - 100 MPH or i'm in a stop and go traffic, my water temp gauge suddenly rises to above 100, about in the middle of 100 - 120(redline) deg. C. but when i drive back to normal speed range, without stopping, between the ranges of 40 - 75 MPH the water temp gauge just drops like a rock to between 80 - 100 deg. C. any idea what wrong with my cooling system??

a) my thermostat is stock 80 deg. C. should i go to 76 deg. C ???
b) do i have a bad fan clutch or fan blade ???
c) do i have a bad water pump ???
d) do i have a bad radiator ???
e) for some reason, my car doesn't have the tube or pipe that goes between
the top front side of the water pomp housing and the front of the
cylinder head. both ends of this piece is almost similar to the ends of the
ALDA banjo bolt except that it is all made of metal. do i need to install
one of those on my cooling system to solve my overheating problem ???

once again my car is a 1985 300D TurboDiesel , California Version.