On one of my ESP's the 5th and 6th fret on the High E sound the same as the 7th.
Also on 15 and 16.

It just happened today, and i have been using this as a practice guitar for 2 years now.

The seventh fret is a little higher than the sixth or fifth, so the string's fretting on that fret when you try and fret on either of the others. Try raising your action, if that doesn't help, you might need a fret job.