I had the exact same problem about a year ago with my 91 300TE.

I replaced the radiator...I now have two spares...thermostat, bought three...new hoses, replaced fan clutch...pulled the hairs out of my head!

Finally I took it to Mike Schulte's place...which is recommended in our forum and he discovered that the cooling fan switch had been plugged in bassackwards!

Now this car runs so good it's scary!

I just took it to Lake Havasu last weekend and the temp gauge did not move one bit from where its supposed to be.

Oh...I'm at 230,000 miles.

Good luck!