Sounds like your getting ready to run out of gas. The idle will raise when running out.
Or....someone adjusted it when it was cold and not operating tempature.
Simply warm it up by riding it. When its completely warmed up, then simply turn the
idle speed knob down a little. IIRC the idle should be set between 1200-1500rpm.
They idle a little bit faster than most bikes.
It the idle is set too low, they tend to stall out when taking off, or studder.
Hope this helps.
I'd also run 2oz of seafoam through a full tank just to make sure the jets are clean.
They may also be varnished causing it to idle high.