Coolant escaping overflow tank

[tnstone]

I have a 2000 Pontiac Sunfire 2.2L. It has 203,000 miles. About four weeks ago, I was driving to work when the car started to overheat. I managed to hobble it home. The water pump was out. After replacing the water pump, the temperature ran a little cooler than normal, especially at speeds of 70 mph. After a while it ran normal and then would raise and lower back to normal several times. Two weeks later, it did not lower and started to overheat again. I got out of the car and noticed coolant had escaped the overflow tank. Got it home. Changed the thermostat (we had bought a thermostat and changed it the first time thinking that it was the problem as the water pump was changed just over a year ago. It was the water pump.) We put the old thermostat back on. The car ran at lower than normal operating temperature. Coolant is still escaping the overflow tank two weeks later. The car is now starting to run at normal operating temperature after idling and will quickly climb a little higher when idling. But as soon as I start moving, it lowers again an stays a little cooler.

I just changed the overflow tank cap because I had a hard time screwing it off and on. It felt like it wasn't threaded right. It still feels like the threading isn't right. Maybe it's the thread on the overflow tank itself?

Any suggestions?