FS98 Planes in FS2K

I know this question was probably beaten to death a couple of years ago, but I didn't have FS2K til just recently. OK, I have installed the Sim and the Patch, SOME of my FS98 Planes fly well in FS2K but MANY of my well behaved FS98 planes are vitually unfliable in FS2K! They are all over the sky and the throttle doesn't respond properly. Is there a fix? I would hate to lose the variety of planes I have and there don't seem to be many of the planes I want available designed exclusively with FS2K in mind. BTW, I am primarily interested in Ultralight and small GA craft. Keep it straight and level,Dave