Hartwig has diagnosed the line of variation (LOV) trouble correctly.
It can be confusing, because the LOV is _usually_ (but not always) in
roughly the same direction as the asteroid's motion. (And both are
usually in the plane of the ecliptic, more or less.) But if you
set a one-hour LOV and compare it to the amount of motion in an hour,
they will usually not agree very well.

Regarding asteroid 15500: the problem here is that you would need
a new MPCORB to locate that object. You mention that you have a 19 June
copy of MPCORB.DAT. Coincidentally, I've got a copy from that date,
too, and it stops at asteroid 15197. You will presumably find that
Guide will "go to" asteroid 15197 (if you've got "Use MPCORB" checked),
but _not_ to asteroid 15198.

Get a new MPCORB, and you'll have the new asteroids, and the problem
ought to go away.

-- Bill