Form Shifts upon Advancing Record

I have a form with a subform and the subform has no scroll bars, record
selector, or navigation buttons. There are essentially two columns of fields,
the left column is on the main form, the right column is on the subform.

The problem is, if I have the subform where I want it, so that its fields
are close to the right edge of the form, when I advance to the next record
while the focus is on one of the subform's fields, it shifts the entire form
(both subform and main form) to the left, leaving a space on the right side.
There are no objects extending into this region in design view, and the shift
hides part of the labels for the left column.

The shift happens only if a field on the subform has the focus, not when a
main form field has the focus.

Any ideas on what's going on and how to correct it?
Thanks
Ray