As was stated above, FS2K is a demanding program (some say too demanding and I'd agree). You should take a look at
www.flightsim.com - there should be info there on tuning.
Make sure you have the latest patch from Microsoft.
If all else fails, give Fly!2K by Terminal Reality a try, although the new version of Fly! is due out soon.
PS - FS2002 has been announced... Either they'll fix the performance problems or we'll all need Pentium IVs!
