Unstable Imaging - Causes?

I've been listening to my Music Hall MMF-5.1 through a Bellari VP129 phono pre for about 1.5 months now, and the whole time I've noticed that the imaging in the trebble likes to shift to the right channel every now and then. The entire soundstage will sound evenly distributed until there is a part in the song with a lot of trebble (i.e. sibilance, cymbals, higher octaves of instruments, etc.) at which point the treble shifts slightly to the right. I've suspected that part of the cause might be that the table and tonearm itself are positioned slightly right of center, and I may be getting some "needle cross talk" (or whatever they call that). I may experiment by putting something in front of the turntable to see if that's the problem, but does anyone else have any ideas as to what may cause this?

Thanks