Background:
Last week I got the low coolant light on. Didn't have time to have it fixed so for a week I drove around adding coolant to the system. I looked under the car and in and out of everything. No huge visible leaks, no white smoke and no burning coolant odor. The other month I installed a water temp gauge. Yesterday I did notice that the temp gauge did work most of its way back out and coolant has been spilling from there. So I tightened it and filled the radiator back up.

Current Issue:
I got the low coolant light on again and the fluid is 4" low in the radiator again. I keep filling it and it keeps getting 4" low. But this only happens every other drive. Some trips the radiator is low, some trips its to the top, but the water temp does read higher than average regardless. I did notice that the overflow tank is getting higher and higher, so my feeling is that the coolant is going to the overflow tank but not returning into the radiator.

I'm guessing I need to screw with my overflow tank. Maybe its clogged or something. Or is there a trick to filling a coolant system and I'm doing it all wrong?

Thanks guys!