speedcooling.com

20 years hmmm. i would say the radiator is clogged probably, at least partially.

you could probably take it out of the car and flush it with water real good, then drain it fill with some radiator cleaner or white vinegar & water, shake it around if you can. be careful the rusty crap that comes out will permanently stain anything lol

flush with water sommore

then do another round with your cleaner

if that doesnt do the trick, then i'd replace the radiator.

i'm pretty sure aluminum isnt "better" cooling it's just lighter weight, so popular with racers. i read somewhere that the copper / brass etc is actually more efficient.

but the main deal is you probably have clogging in there. especially if it use to run fine just like it is, and this is a new problem.