I had a similar problem and it turned out to be the Idle Speed control valve not working ( big round shiny cylinder thing just after the throttle body on the 500)

take it out and soak the end in petrol overnight, (just the end not the whole thing) and then gently move the flap valve inside up and down lightly with a thin screwdriver to free it up.

or it could be the blue temp sender at the back of the head by the dipstick