I think you have answered you own question.
The longer you have owned the car the hotter it seems to run.
I bet you have some corrosion build up that is restricting water flow.
I think you are on the right path here. A good flushing of the system may solve the problem.
In the end, the radiator may need replaced (being that it's old and has already been worked over), but I like to at least try to fix a problem without tossing a fist full of cash at a car.
We haven't mentioned tuning. Something like being lean on the top end (65-70 mph with 3.91 gears should have the RPMs up a bit) could cause excessive heat.
