Semi-overheating?

[CoBeerToad]

Needle is always on the 8. Always.

I'm driving the 60 mile commute home with the top down and enjoying myself. I got stuck on an exit ramp to another expressway and decided the car is not as much fun when it's standing still, so I decide to stay on the expressway. Here comes some fun. Merging into 60-70mph traffic from a standstill. The P-car normally has no issues with this. I get on it and somewhere after the first shift into second, I hear a thump. Not enough for me to back down, but enough for me to notice. When I settle down, I notice my needle is on the zero. See above. It's never on the zero, so I start to freak out a little bit. Traffic is flowing fine and the needle stays there. Traffic backs up and the needle stays there. 40 miles later, I arrive home and the needle is still there. I check the coolant after it cools down, and it is low. About 1/2 gallon low.

Next weekend I buy some distilled water (I have no idea what coolant is in there and I'm making plans to flush it in the next couple weeks). I fill up the reservoir and idle the car until it hits operating temperature. Then I run the A/C. Coolant level does not move. I decide to take a spin and put some fuel in it and the needle sits on zero. Coolant level is unchanged and no matter how hard I beat on it, it stays on zero.

Anyone have any ideas so I don't have to drive a car with a measly sunroof all summer?

The cap is the wrong (old) type and it is on order as of 5 minutes from now. Could that be the source of my problem?