Phil, I agree that in many, many instances, the OEM part is the best. I belong to a car forum for AC Cobra replica owners, and again and again, I see problems pop up from guys using aftermarket items. OEM may cost more, but I believe you get what you pay for. Nowadays unfortunately, you have to be aware of even dealers selling aftermarket parts. Ask them if they have both OEM and aftermarket, before buying the item. Now with that said, there are times where an aftermarket part works wonderful, and can save you serious money. Some parts are made for the OEM, but are repackaged and sold aftermarket as well..roll of the dice..
Curious flyfisherman, as to the ratio of anti-freeze you are running? Anti-freeze also works as a coolant, so if your ratio is off, that could make a difference.
Some of the fellows running high performance vehicles highly recommend adding a product called "Water Wetter" for better cooling. Might be worth looking into..
If it's the original 92 rad..the cores may be plugged up, so ultimately a new rad may be the only solution.