Hi Phil,
PMFJI, but I think I might be able to help.
I agree that you have a dangling object reference somewhere which
is preventing the form from being destroyed. The Destroy code is
running (VFP just doesn't finish the cleanup). This could explain
your error with This.oHost. I suspect that This.oHost has been set
to NULL by the Destroy logic. It also explains the greying out of
the close button (VFP's behavior when it fails to destroy a form).

The error that you are getting:(from VFP5 help file)
ERROR #1737:
name cannot store a value because it is not a property
Only properties can store values.

makes me suspect that _MOUSEOVERFX1 is not an object property at all,
but rather an object that was added via the AddObject method of your
form. Try ThisForm.RemoveObject("_MOUSEOVERFX1"). It should at least
cause the object to go away cleanly and might cure your problem.
HTH,
Ned