first thing you should do is get the car up to temp. (*the middle line is 100C) then immediately pull over, and shut down. pop the hood, and see if the fan spins easily, or it's stiff. if easy, your fan clutch is failing.
next as a temp measure, get a good solid jumper wire, and place it between the two phillips screws on the large resistor behind the driver's headlight.
that'll allow the aux fan to engage at full speed when called on.
lemme guess, 134a freon in your a/c?
this is what mine was doing, and I found these issues.
I have a 93 parts car, so I'm gonna pull several parts and upgrade my TD!