I had a 1994 eldo with same promblem. 99% of the time its the head gaskets have gone bad. Take it to caddy the can run test. Warning very expensive job.
While it can be
the head gasket, I do not think the probability of that is anywhere near 99%.
There are many factors here we do not know. Is the system losing coolant? Has a pressure/leak test been done? I'd check the lower radiator hose; it might have gotten soft and collapses when coolant flows through it rapidly at higher speeds. The water pump itself can be bad, or its belt is slipping...
I'd start with the simple things, diagnose it accurately and go from there.