Hi,
I've had this problem many times. I'm not sure why this happens. The way I've been sometimes able to fix this in the past is by defining the digitizing window in more of a diamond shape, as opposed to rectangular.
Often, sometimes, it helps if I define a small part of the map as the digitizing window (also in diamond shape), as opposed to getting the entire map in the digitizing window.
If this works for you, make sure to do your editing using the mouse, not the digitizer. This way you won't need to deal with defining your window on the the digitizer.
I hope this helps. If the above doesn't make sense, I'll try to explain it more clearly. Or just give me a call.
Good luck,
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Christine Weiss
NYC Dept. Parks & Recreation
Natural Resources Group
cmw@nycparks.gov
(212) 360-2749