Here's the response I got from Support at X-
10. They replied within two days!
"The first thing that you want to do is
check that the bulbs used in the PR511 are
not halogen or CFL, as that will cause the
lights to stay on. If you are not sure what
kind of bulbs you are using, try switching
to standard indoor bulbs that you can see
the filament inside.
If turned on by a command from an X10
Controller, the Motion Monitor stays on
until turned off from and X10 Controller, if
necessary send an Off command to the Motion
Monitor from an X10 controller, returning
the Motion Monitor to normal cycling mode.
Also, if the Motion Monitor is accidentally
powered off (from a wall switch that
controls the power to it) while the
floodlights are on, it will stay on when the
power is reapplied. After dark, (depending
on the position of the dusk setting) trigger
the Motion Monitor by walking in front of
it, then after the delay time you’ve set the
floodlights will turn off and return to
normal operation."
The second fix worked for me, and I do have
Halgoen bulbs..