What temp are you calling overheat? Up in the red? Is it always, or only when sitting still? Is the electric fan running when it is hot? Also, check and feel if the radiator is hot all over or only at the bottom at the inlet. It could be clogging up....it should be hot all over it when the car is hot. The side furthest from the inlet, and on the bottom will clog first. Feel there.
Did you convert to R134 from R12 in your a/c? If the pressure switch is not changed on the dessicant chamber, the electric fan will not come on with the a/c. The pressure for R134 is not enough to trigger the R12 switch. So overheat when you use the a/c.
Take look and post more info. You will get it fixed!! Good luck!
1989 560SEL (172k)
1989 325IC (122K)
2004 Suzuki Volusia LE (3500 miles)
2005 Yamaha Road Star (20 K miles)