weird single notes sounding sour problem
 

My MM Sub 4 has a problem on the higher registers. From about the 14th fret up, when I play a single note, it sounds like a sour chord. Almost as if I were playing a D and then another D note within the same octave, but slightly sharp or flat. I thought it may have been the cheap strings I put on going dead, but it's doing the exact same thing with a brand new set of strings. It's intonated perfectly, and the neck is dead straight. I've heard this on two guitars before, and it went away with fresh strings, so I'm a little worried.

Can anyone offer some insight as to what this could be?

You may need to put a little relief in the neck. Dead straight neck isn't always the best way to set it, although that's how I have most of mine. I'd loosen it a quarter turn and see what happens. Or raise the action a wee bit. Or both. If it keeps happening then someonenelse is gonna have to help you. :)

Well, I found the problem. Pickup was set too high. All is well now.