Recently I was driving on the freeway and I noticed that my temp guage went as high as 120c but not into the red... the temp. outside was not hot maybe around 78*. I replaced the themostat and flushed the radiator and added new coolant... then took it out on the freeway again with outside temp at around 60* and the needle went from 40-89 in about 4-5 mins but never went above 90...
Someone made the remark that the radiator could be losing pressure around the tank seal. It is not losing coolant. I have not had to put coolant in the resevoir in over 2 yrs.
Is it possible for a temperature rise to be caused by a radiator that is losing pressure and how do I determine if that is my problem. Could there be anyother problem I am overlooking?

1982 300SD Turbo