SOLUTION: So after spending the entire weekend trying to figure this out, I took it to a local mechanic today. I explained that I had replaced the coolant reservoir/cap, thermostat and waterpump. They had the car literally all day! They pressure tested the cooling system (no leaks found), they did a flow test on the radiator and engine block (low flow at block), they tested for head gasket weakness/blown (passed), they removed the thermostat to verify no damage....then they did a test on the surge cap on the reservoir (which was brand new) and that was it! The cap was not holding pressure despite it being brand new. So...the cost to test everything and replace the cap plus coolant was $188. Stupid expensive for such an inexpensive item, but they did have it all day testing this and that. I was prepared for a $1,500-$2,000 blown head gasket so the $188 actually felt like a win. Anyway, thanks for the replies. I hope this helps someone in the future!