Power valve did it stick?

[mattdenn]

Hi Guys
07 gas gas ec250, owned for a year and always run and been reliable. At the weekend whilst riding on a uphill gradient the bike note changed and i lost power. The engine continued to run but the exhuast sounded rattly and although the bike still moved it seemed to have no power at all. After stooping and starting the bike seemed fine again. about an hour later it done the same thing. with a good handfull of throttle it cleared itself again. it then run the rest of the day o.k. anyone got any ideas what might be going on?
thanks