We had similar - trundle around perfectly fine, in town or at speed - all OK.
But climb a hill of any length (which is half our driving)... water would spit out the overflow, and even dribble out the cap. Often it would even foam up (I use antifreeze).
We tried commercial radiator cleaner 3x and immense amounts of flushing - the colour of the stuff coming out was nasty... but it was the same when climbing a hill
.
We took the radiator to an old school radiator shop in town, who removed the tanks, rodded everything out and kept the junk... there was easily 3/4 to 1 cup of crud in the container he collected it in!
Since then it's never done it again, and we've installed a 160F thermostat and gauge.
There is a VERY steep hill getting to a friends house: It has an elevation gain of 351 feet in just over 1000 yards (there are a couple of
small downhills in there too!), with a peak gradient of 29% and lots of corners
Even after the fairly hilly and twisty coastal drive out there (well warmed up to the 160F thermostat temperature), It chugged up there with no temperature concerns in the peak of our humid summer! I was getting worried about how
I was going to manage double clutching down into 1st gear while climbing the steepest part/corner - but we JUST managed to hold in second gear and the temp never climbed past 180F on the worst part with a lot of throttle at maybe 1000 RPM at a guess.
I'd recommend sorting the radiator really well
