My 99 Subaru legacy sedan. 2.5 engine. The car will run fine for over a month with heat gauge on normal. Then all at once the heater throws cold air and the heat gauge starts climbing to hot real fast. When I stop and raise the hood the engine is so cold I can lay my hand on it and the radiator. I had the thermostat replaced about 6 months ago when it did this same thing on a hot day. When it does this the overflow jug goes full and the radiator goes low and even after sitting for two days in the cold temps if you take the lid loose or radiator cap it immediately pops pressure out of them. Could the water pump be plugged up or going bad? It doesn't show any leaks any where or noises. I am very aware of car repairing as my dad was a mechanic for about 45 yrs. When I was younger with the older cars.

That is typical of air blockage. Bleed the system. Also realize that the 2.5 Boxer has had issues with head gaskets and at this age I would do a leakdown test. Leaking head gaskets could cause this problem too.