Why are the vocals on some records hidden behind the music on my system?

Yogiboy and Geoffkait, I think you may be inadvertently confusing the OP.  There is a difference between issues related to "system phase" (for want of a better word), which I use to refer to the phase in both channels simultaneously, and the issue of having one channel 180 degrees out of phase with the other.  The latter wreaks havoc on imaging, for sure.  The former has a subtle effect (if the channels are in phase with each other and you then switch the phase in BOTH channels by 180 degrees) heard by some but not by all listeners. 

I would say off the top of my bald head that it is impossible to cut an LP such that one channel would be reproduced 180 degrees out of phase with the other.  That can happen at any downstream point in the circuit after transduction, but not a fault of the LP, just based on how a stereo signal is encoded on an LP.  The cartridge itself could be internally miswired to create such a problem (which I don't think is happening here), but the LP cannot be at fault.  (Please, anyone, if I am wrong in this belief, correct me.  No problem.)  Perhaps you guys are attributing the OP's dilemma to system phase (i.e., the two channels are in phase with each other but phase is opposite to what sounds best), which would be the easiest thing in the world to check; switch the leads from hot to ground and vice-versa on BOTH speakers.

Scooby, You say above that you reversed the speaker cables, per my suggestion here.  Did you do it at both speakers or only at one of the two speakers?  

Just to complete the list of suggestions, so that every possible nook and cranny of LP playing has been covered, try playing with VTA/SRA by raising or lowering the tonearm pivot a tiny bit.  But like I wrote up above, I really think you are hearing a characteristic that may be inherent to this particular cartridge, albeit you probably can change the soundstaging or tonal balance or whatever you want to call this problem at least a bit by making very small changes here and there.  Good luck.

Do you know the phono gain via your MM inputs? It is (remotely) possible that the gain is marginal for a 2.2mV cartridge output, I suppose.  But it would help to have some more details regarding your phono stage; does it have separate MM and MC inputs, as Czarivey suggests?

Other than that, I think you may have no problem; in my experience some cartridges do tend to submerge the vocalist in favor of the instrumentalists, compared to others.  This can actually be a virtue of the cartridge, in the case where you are not listening to a single, stage-center vocal LP.  For example, I found that my Stanton 980LZS tended to paint that sort of picture.  But the Stanton excels at bringing forth internal musical threads within a group of instruments.  It's certainly not broken.  The test for this explanation, of course, is to try another cartridge.

Nothing that you wrote suggests to me that the two channels are necessarily out of phase with each other, but be sure of that, too.  Nothing at all suggests a problem with turntable speed stability; I don't know where that came from.