Guys i would need some help for a friend of mine who owns a 3 year old Saito 50.

Well this is what happenes....It starts up with full rpm but as time passes it loses power/rmp. after a few minutes the max rpm is so low it cant even pull up a 20 sized aircraft. we have tried tuning and all but still no improvement(cant be the tuning as the engine has good rmp when the engine is just started)
But the engine does not cut off but still runs with very little rpm(like its idling at open throttle)

SO what can be the problem? And what should be done.
Thanks for the replies and help guys.