Are you using orange or yellow Prestone?I just use prestone premix 50/50. I've drained and filled my system a few times at this point though, no issues have reared up yet.
I just run their "safe for all makes/models" yellow. Coolant system has been dumped and filled 3 or 4 times already with it.Are you using orange or yellow Prestone?
I originally used orange Prestone to top mine off, but gave that jug to friend with a GM. The yellow is same main chemistry as Ford, although some of the acid and inhibitors are different.