2000 xp throttle surge

[bgp429]

Have owned this XP for over 10 years. Last year it started to develop a RPM surge between 4200 rpm and 4800 rpm. Problem is that is where I like to cruise so I have to constantly play with the throttle. XP is completely stock. Sent the carbs to repair shop and they were rebuilt. He really didn't find anything wrong, no corrosion, or other issue. Runs fine otherwise but that problem still exists. Any suggestions? Thanks