Unstable Imaging - Causes?

..an easy way to check for proper azimuth is to use a small mirror (preferably one with the reflective side on the viewing side - a camera repair shop is one place to get such a mirror). Set the mirror on the turntable with the arm resting on the mirror, close one eye... Looking STRAIGHT AND HEAD ON, adjust the headshell to be as perpendicular as possible to the reflection. You're really trying to get the stylus perpenducular, but oft times, its hard to see, and mostly, cartridges are manufactured close enough for you to use the cartridge body as a guide.

Jwglista: The MMf 5.1 as the bellari are un-familiar to me, anyway here are my thoughts about:

+++++ " 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. " +++++

Other than you make exactly what Newbee already posted ( on the VTF subject you could try to set-up at the upper VTF limit of your cartridge specs and obviously that the tonearm is flat leveled. ) ) is to check that the tonearm/headshell wires are connected in right way to the cartridge: red one with red cartridge pin, etc, etc, then check if the RCAs tonearm outputs coincide with those tonearm/headshell wires: red wire with right output signal, white one with left output signal, green one with right ground output, etc, etc.

Other than these could be on a cartridge out of target or something wrong on the Bellari.

Regards and enjoy the music.
Raul.

Well thanks for the all the suggestions, but I still don't seem to have things exactly right. I tried playing with the azimuth, which seemed to help, but it is very difficult to accurately adjust on the MMF-5.1. I also tried increasing the VTF per Raul's suggestion, which didn't seem to help with the imaging. As for the mirror thing suggested by Stringring, I did try that but used the little mirror on the back of my stylus brush; I have it to a point now where to the naked eye, it looks very straight. But who knows if it's exactly correct. I also tried switching from the Bellari VP129 back to my NAD T162 preamp with built-in phono stage. The sound was much more sterile and congested, but the imaging problem seemed to be lessened. I later realized that this is probably just because it was masking the issue, because I could still faintly hear the shifts.

I've noticed that a lot of the imaging shifts also occur when trying to reproduce the room reflections of the recording venue. The unstable imaging becomes most audible when listening to room reverb and those room reflections.

Could all of this simply be the result of owning a budget turntable/tonearm/cart? I've been considering buying a test record, preferably from e-bay because new copies seem to cost a lot...

Does this high frequency shift mainly on loud levels? If so it is entirely possible that your cartridge or the combo of it and the arm is causing a mistracking which causes, for some ellusive reason I can't comprehend, some uneven and excessive highs in one channel OR a reduction in the highs in the other channel which might well cause the same effect. I think getting a test disc (at what ever cost - so long as it is of value) is a far better idea than just getting higher quality TT's, arms, and cartridges, even though I suspect that maybe where you end up.

Forgive me for being redundant, but have you ruled out speakers and speaker placement as being an issue by at least reversing cables at the phono so the distortions which now appear on the right channel will appear in the left channel if the problem is associated with your TT set up?