Alright, so I used to have a Squier Fat strat
and if you tuned the strings correctly,
it'd be in tune.
but if you lay a finger on the frets around the middle of the neck, its out of tune
but when you play the open string, its in tune
for that reason, i bought an epiphone G-400 very recently.
loved it right away.
this is my 3rd week or 4th week with this.
And I'm starting to get the same problem.
Already! In only less than a month usage.
What is this problem?
How do I prevent it?
How do I fix it?
and Im starting to get some annoying buzzes..
I tuned the strings correctly.
and whenever i put a finger around the 12th fret,
that notes out of tune
even though the open string remains in tune.
Okay, maybe I used the wrong word >_>
maybe not intonation,
but whatever this problem is,
how do you fix it..
you fix it by correcting the intonation
Tuning is different than intonation.

To fix the intonation, check the tuning of the 12th fret compared to the first fret and the 12th fret harmonic compared to the 12th fret.

If the note is flat, move the saddle towards the neck and if its sharp move it away.
I've developed a complex where everytime I hear a Lamb of God song, I burst out laughing

My 7 String V build
My Main Guitars:
Kramer Striker FR-2027SM 7 String
BC Rich Afterburner Warlock
Washburn Xb100 Bass
My Effect(s)/Misc:
Digitech RP350
The tuning up the neck is called the intonation. If that's off it make things sound out of tune even if your open strings are tuned correctly. Didn't you read the link?

Also when you say you're getting buzzes, are yout talking about fret buzz or buzz/hum through an amp?
Thanks for the link mate,
and I posted that reply before I read yours, my bad.
And for buzzing, it's fret buzz
Ah ok, do you understand it now?

For the fret buzz raise the action some, there should be something on that site I linked you to that shows you how to raise it, but if not look here http://projectguitar.com/tut/action.htm

Keep in mind that if you raise/lower, you'll have to adjust the intonation again.