2002 Mustang GT idle issue

I have a 2002 Mustang GT 5 speed. Yesterday when I was driving at about 55 mph I put it into neutral just to coast. My car just surged down in rpms and stalled. It started right back up with no problem in the turning lane. Today, I took it to test it out and see how it was running. It never stalled on me, but when I get up to higher speeds (around 50-70 mph) and put it in neutral, my rpms will drop down around 500 to 400 and move up and down very strangely. It only happens at higher speeds. Can anyone explain this?