2004 Scion xA Overflow bottle

The overflow bottle constantly overfills and causes antifreeze to spray out through the cap on the overflor bottle. I recently replaced the water pump and the radiator cap with Scion certified parts. I also noticed that the overheat temp light on the instrument cluster illuminates for a brief moment and then truns off. When this occured the cooling fan started up but never shut itself down like it's supose to. I was thinking that the thermostat had failed, but if it fail open the the engine would never reach ooperating tem, but if it fail closed the engine would over heat. What am I missing? Thanks much.