Thanks for the info everyone. Forgot to mention that FS2004 is being run here on Windows XP and Vista (two sets of install discs being used), and the problem cropped up within the past couple of weeks on XP, as this is the OS that I ususally use (Vista being mainly as backup). Anyhow, last evening while backing everything up on Vista, I tested FS9 there to see if the same problem occurred. Strangely, it did not. All the aircraft appeared on the radar and remained in flight while being "tailed", e.g., none disappeared despite being followed for consisderable distances. Looks like an uninstall/reinstall may be in order for XP.
p.s. - The flightplan mods involve adjusted destinations to a custom base which has ample parking of many types, and all use IFR. Many custom AFCADS have also been added to accommodate parking needs. I took care to ensure the cruise speed was correct for each aircraft, as well as verifying that enough time remained at rest between destinations. Pretty odd but am glad that the problem appears to be probably with my install and not with the flightplans/aircraft used, as this project took quite awhile to put together.
