Filthy radiators definietely affect the colling system. The left one seems to be a little bit dirty.
Maybe too much farm roads on you daily routes.
If the car was slightly overheated while driving the overpressure valve will open at around 1.3 bar. So you loose coolant fluid and the colling system stays save.
Just another idea. Very old coolant fluid can get instable (hope that is the right term) and rusty. This coolant fluid will be less efficient. Had that problem myself with a 12 year old car, where the fluid never had benn changed. Drained the system - a lot of rust. Flushed the system with light pressure and fresh water - still a lot of rust. Flushed the system until the water was clean. Than filled it up with new destillated water and cooling fluid. No more cooling problems. But it wasn't a Boxster. So it was a lot easier to get the whole system flushed. Maybe this isn't a Porsche problem, because of the aluminium engine block - the car i had had an "iron" engine, but maybe it is worth a try.