Only thing I can think of is that your headgasket is going bad...
I never had any cooling problems with my B16 Turbo...

This is what I think is happening... You're headgasket may be slightly leaking, introducing combustion pressure into the cooling system, increasing the pressure beyond the rated cap, and causing the excess pressurized coolant to enter the overflow tank.
Can you see any bubbles in the overflow tank or radiator with the cap off?
What about an oily film on the surface of the coolant? Look in the radiator with the cap off & a flashlight.

Best bet is to do a compression test then a leak-down test to determine if in-fact you are losing pressure into the cooling system...

Good luck!