Thanks jetflite, interesting what you have written especially about wing vortices stabalizing 498ft to 898 below the flightpath.
The a/c has come back from maintence no obvious problem was found with the AP but it was only a prelimary check. This and your suggestions make me believe that it probaly was wake turbulence. All though saying this the a/c still rocks its wings on AP so it will have a more extensive examination shortly.
Once again thanks for all replies,
Mooneyboy