Hi mate,
Could be that after the winter storage the clutch plates are sticked together. Does the clutch feel "harder"?
I would suggest you drain the oil, open the clutch, and take out all disks, clean each thoroughly and put in oil one by one, and reassemble. Beware to keep the order as the last disc towards the crank shaft is thicker. This should cure the issue.
Had similar problem before.

Keep us posted.

Regards

__________________
that is how i feel about my XTR
Mods so far:
Hyperpro front springs; Hyperpro rear shock spring; modified air box; LED tail light; LED indicators + new relay; YZ pegs; Lucas aluminum bar; brisk spark plug; snorkel removed; DNA stage 1; second hand DynoJet O2 mod; crash bars; front and rear steel braided brake lines; soft EBC brake pads; mitas 07 tyres, 2in1 Metal Mule exhaust, Blocked AIS, tall OEM screen, OTR bash plate