I do have anti-freeze in there with water+water wetter. Like Rob said I am planning to switch do a cobra pulley for added cooling.
This didn't happen during idle only, it happened after a quick drive around the block after idling for about 30 min.
Then I think you are not getting enough flow...Measure Your pulley before I get there today...If it is larger than the one from the Green Stang, we can swap that one over and see if it helps before you spend $$ on a Cobra one.
RC