Fred,
I have the same type of drift and mine is cyclic.
I have never measured it but it's down to my reactive soils
and shallow dome slabs.
More about it here in this thread.
http://www.iceinspace.com.au/forum/s...32&postcount=2
I have used a laser for years mounted on my GEM and
it shines on an X on the nearby house.
I can see it drift over winter and summer months.
A quick re-set gets most of it zeroed out.
Another night with Drift Explorer and it's gone.
Steve