that sounds like my problem before which i couldn't find out, til my block threw a rod 8D

i'm thinking when yoru temp was going up and down, you were prolly low on rad fluid, til you refilled it. I'm thinking it went something like this, example when your car was in op temp, while intraffic you temp goes up, until the vehicle was in motion and air was flowing thorugh your radiator it went down. SOMETHING LIKE THat.. that was 99.9 percent that it was just yoru rad fluid running low.

and now if everything is going all good but your reservoir tank is overflowing... I didn't understand that myself as I had this problem, And i've taken this to a few shops to troubleshoot. and they couldn't give me an answer until 1 month later my block threw a rod.. so everything above was taken kared of with the rod through the block 8D.. Chances are your headgasket might be going out, and or prolly a little knick knack onthe headgasket .. it's prolly busted but not to the point where it's damaged yet.. so hopefully you get this taken kare of. Otherwise you'll be in my shoes pretty soon 8D