1998 C70 Hardtop, 220k miles

Had some significant coolant loss from a bad seam on the side-tank radiator.
Replaced with a new Nissen's unit. Installed new top hose. Took me 4 hot sweaty hours to replace. No leaks, refilled with approx 1 3/4 gallons of dexcool/distilled h20.

All seemed well on Monday.

Tuesday, it blew the intercooler hose off on the interstate. Car began to run very badly. fixed with a new clamp... which I actually tightened this time :\

Wednesday (very hot here) the AC began to work poorly coolant began to boil out of the expansion tank cap. Low coolant light came on. Coolant boiled out at home, and again when picking up kids from school... and again that evening. Replaced the green cap expansion tank with another spare green cap.

Thursday *much* cooler. I topped up the expansion tank *again* in AM with distilled. Coolant smell in cabin. Slow boil from the cap after parking. I can see the hot coolant boiling in from the thermostat bypass on the housing. Little actual loss, but very concerned.

Something is wrong. Thinking it could be the thermostat, or expansion tank, or caps. Went ahead an ordered $150 worth of new parts (Thermostat, OEM expansion tank, hoses, sensor, etc) from FCP Euro.

My working theory is that the new radiator is putting OEM pressures on the rest of the 220k-old system. Thinking it's the thermostat, but really unsure what's going on here. One minute the expansion tank is empty, the next hot coolant rushes back in and it fills to the brim before overflowing.

Any ideas what's happening here?

Could my water pump be an issue? I'm tracking odd noises from the accessory belt area - thinking it could be the power steering pump... high miles car, figure it could be a lot of things...