Gen II Swap -- Odd Coolant Issue -- HELP! Options

[Fastbird]

Noticed it last week, progressed to a head just now when my wife got home. Last week when I'd get home I'd hear some gurgling coming from the cooling system. Last night when my wife got home it was steaming out of the vent in the overflow resevoir. Today she just parked in the driveway and there's a huge puddle of boiling coolant under the car that's coming from that overflow resevoir vent.

The car is NOT overheating. It's running fine. So that kind of rules out the thermostat as if it was stuck closed there'd be obvious indicators. No leaks or cracks noted. Oil is fine in color as is coolant.

Thoughts?

[Fastbird]

Nope. Nothing there was changed. I did find last night that the spring loaded center section of the radiator cap is broken/not attached, but I didn't think that would cause it as the cap still seems to seal. Heading out for a new on momentarily.