Why did you add water when it overheated? Was it distilled water I hope? Also was the level low when you added?
Check everything mentioned above, it's also possible adding water to an otherwise ok/full system could have jacked things up.
Plan on replacing your wp and tstat - at 70k miles, if it hasn't died yet, it will soon anyways, but this is most likely your issue. You'll flush and electronically bleed your coolant as part of this procedure as well.