what is going on here?

if you look at the low speed needle, around that is a brass collar, behind that the horn that goes to the throttle servo. This thing was running insane yesterday, now, this horn just goes wherever it likes. It'll center about halfway above the needle sometimes, at other times it'll center at about the 3 o'clock position, no particular place really, just wherever it wants. What is loose? how do I tighten it?

thanks