Thank you both for the replies. Sounds like both of you have had your own odd ball graphics issues, and the lack of additional replies tells me this particular issue is specific to my system ( on two different graphics cards really baffles me tho...)
Other odd notes: If I zoom (Vert or Hor) the phantom horizontal lines stay put...so it does act like a graphic remnant that isn't refreshed.
However..If I return to my "all tracks" view and
1)
hide all folders2) select/focus on one track to edit
the phantom horizontal lines disappear ( because folders *weren't* visible in prior screen)
For anyone else that ever finds this thread with a similar issue, my workaround:
I save my "all tracks" view to Screenset 1
I save a frequently edited track isolated to Screenset 2.
Switching views this way (instead of "show selected track" shortcut) DOES NOT result in the phantom horizontal lines.
If the lines are really hampering your editing space, this is a reasonably workaround :)