I'm sorry, I'm not familiar with the clocks on the '05 Rockets. If like the one I purchased NOS from Triumph, one that mounts on the handle bar, it is a mechanical clock with electric windup solenoid. It that case, the varying voltage is not going to be the source of the problem (as long as the battery is charged).
I didn't want to fool with it, and returned it. BUT, had I wanted to keep it, I know a place that still works on watches and such, and I would have taken it there.
I'm guessing, your clock is just "tired". Original lubricant has dried, and no longer effective, and internal resistance has increased to the point the drive spring cannot overcome it, and the clock slows -- noticeably in your case.
What I did - since the cost was about the same, was go for a Martin Instruments clock, and am happy with it.