Hi Jona
I've had the same problem and let my son continue to ride.  I had to get a new engine fitted after 6 weeks as the piston rings broke up and scored the insde of the engine.
The same symptoms started again, but this time I've had a good look around the bike.  The air filter to the carburetor is clogged full of mud.  I have cleaned that and the bike seems to run fine now.
I must admit though, my advice is to find a local bike shop and get them to check it over for you.  I have used one locally and they are very reasonably priced and did a great job
                
 Alistair
                
 April 2005