Rega Planar2 Tonearm

The problem I've been having is that the tonearm on my P2 (1997 version, glass platter) tends to drift outward when I cue to play a record. At the beginning of a record, when I cue the tonearm very slowly it comes down close to the first track, but it is tricky to find tracks in the middle of an lp. I tried to re-balance my tonearm, setting it to "float" above an lp, as recommended in the Planar2 instruction sheet, but found it impossible as the tonearm drifts off the platter. I also checked and determined that the plinth(?) of my Rega, and the shelf it's on, are both perfectly level.

Any advice would be greatly appreciated. Thanks!
You know the little green slider on top of hte base of the arm? That's the anitskate. Try setting it to "0" when you adjust the counterweight. Get a test record then set the antiskate slider according to the test record instructions. Do this after you set the tracking force with the counterweight. Please see audioasylum's FAQ --> tonearm/cart setup pages, so you can get the idea of how all the setup dimensions work. If you know all this already, sorry.