My 1990 Lexus ES250 (aka Camry V6) has had an annoying but not fatal cooling problem for a couple of years, without any significant change.
The car runs flawlessly if the outside temperature is no higher than say the mid 80's or maybe a bit higher, but when it gets into the 90's or higher, it's really marginal.
Usually the first thing that happens when it's hot outside and I drive up a shallow grade (after a half hour at freeway speed) is that the aircon stops cooling. The temperature gauge remains at its normal point, where it's run within a couple of needle widths since the car was new. I get off onto (flat) surface streets about that time on the way home, and in about 5 minutes the aircon usually comes back on. When I pull in to the garage, if I shut down but turn the ignition back on (the Lexus fans don't stay on with it off) the fans stay on high for five or ten minutes, then drop to low for a couple of minutes.
Today it was near 100, and the aircon actually shut down much earlier, on a section of the road that you'd have to know to realize it was climbing a bit. It came back on after a few minutes. Then at the hill it shut down and for the first time I can remember the temperature gauge climbed to about halfway between the normal point and the red line. I took it relatively easy up the hill, but it stayed at that temperature the rest of the way home. I left the fans running and it cooled off fairly quickly. I figured I'd lost a lot of water, but in fact it didn't seem so, although I refilled the expansion bottle after while.
The water pump was replaced a few years ago, and at least isn't leaking. The thermostat is just a few months old, genuine Toyota part fwiw. No water in the oil, and mechanic thinks engine problem is unlikely since he thinks gasket/block problem would show the problem even at normal outside temperatures, not just when hot.
The only other symptom I can think of is when I turn the heater on there's a "waterfall" sound especially if I accelerate up a ramp.
My present prime suspect is the radiator, for no good reason other than it was cheap.. It's an aftermarket replacement I got several years ago when the plastic tank on the oem cracked. I'm thinking perhaps I shouldn't have bought it from the guy who gave me the cheapest price on the phone. It was okay for a while, of course, otherwise I would have taken it back and had a word with the installer.
Thanks for your comments.