Epiphone strange off tune problem

Hi,

Last week, I bought a new Epiphone Les Paul Standard that was adjusted by a guitar tech.

The only problem I have with this guitar is when I play, for example, the G string 5th fret and B string 6th fret simultaneously the "chord" sounds on tune, but if I play G string 5th fret and B string 5th fret it sounds totally off tune. If I tune it accordingly, then the first combination of the 2 notes is ok but the 2nd isn't.

The problem is even more noticeable when I put some distortion. My other guitar (Yamaha Pacifica 312) doesn't have that problem.

I'd like to have some of your ideas on that and see if it can be solved.

Thanks