bond@ From the information given i'm afraid that what I promulgated in my reply was correct. Some people however have seen this problem in the past - I never have thank goodness. Please give more information and more people will be along that can give advice and possibly a cure.
1. Win XP or Win 7.
2. Processor type/speed
3. Memory - how much/speed.
4. FS 2004 or FS X.
5. What version of the 767 are you running, is it Download or Disc.
And anything else you can think of such as addon's or specific routes this happens on or length of time into the flight.
All information will help.

It's certainly not time to decommission it yet.