Weird that you found it is sitting flush, shouldn't be crawling like that then. Those three bolt ends on the back are designed to wear, so if washers are removed and it starts squealing or crawling like you're describing then I'd replace them and start with the washers installed again, but in your case you say the belt is flush so that shouldn't be the issue. For your knowledge more bolt depth holds the sheaves apart farther and lowers the belt (washers removed), less bolt depth puts the sheaves closer and belt sits higher.
Put the sled on a stand and run the track some (safely!). This will do two things: It'll limber the track up and show you how much it is turning/creeping at idle; and when you rev it up and then let off, let the rpms come back down and the track slow back down without hitting the brake. This will let the belt in the secondary return to it deflection position naturally and for sure show you if it's flush are a bit above/below flush. If the belt isn't sitting more than an 1/8" high in the secondary, then something else is going on. Like mentioned by Viktory, then check for proper belt and center to center.