Ok these are long shots: check to see if water pump is ok...could be belt, pully or pump itself. Since this happen once already and a fill up of coolant took care of it for a while, I'd guess the belt slips and then you overheat, coolant backs out in overflow.
Another possiblity is thermostat is not opening up on occasion.
Lastly it's possible some debris is clogging up the flow, in which case you'd need to flush it out.
Also can you get a read out of your codes, I hear Auto zone will do it for you.
I'm stretching the theory here, so anyone with more experience please keep me honest. Anyhow hope it puts you on the right track.
Another possiblity is thermostat is not opening up on occasion.
Lastly it's possible some debris is clogging up the flow, in which case you'd need to flush it out.
Also can you get a read out of your codes, I hear Auto zone will do it for you.
I'm stretching the theory here, so anyone with more experience please keep me honest. Anyhow hope it puts you on the right track.