Undo reset when switching between viewsets

[Anonymous]

my colleague is having problems with the Undo function - when he changes from one viewset to another it resets the Undo - i.e disables Undo as if he had saved. I've checked autosave options but don't think it is that as autosave shouldn't override Undo.

Undo limit is set to 20 steps.
Autosave was set at 10 steps but is now 20steps with no change.

Details:

AC10.1183
iMac 4
OSX
2Gb Ram
64Mb on-board graphics

Any ideas?
Thanks
Mark RR