Jon,,, sounds like your throttle cable is binding someplace. Pull the tank and check.
I had this problem for a bit (intermittently), when I first got the bike. Broke a couple of the OEM cables, (and the soldered "barrel" that slips into the rotor). Get one of the heavy duty, American made ones that Classic sells, or go by Gonzales in Tacoma and see if he'll make one up for you.
When changing out one of the broken ones, I pulled the throttle completely free of routing it through the 'caskette,' and that pretty much cleared that one up.
(A few too many cables and wires stuffed in that thing altogether).
One way to determine if binding in the caskette is the problem; start the bike, idling, and move the steering head from hard over to hard over. If it binds, it should speed up slightly, hang there even.
I'm considering removing the caskette altogether, going with a more custom look. And better engineered.
Pull that bulb out and you'll see what I mean.
Break that thing in and we'll run up to Ashford someday soon.
Steve J. (Also in Puyallup)