so my gibson sg won't stay in tune properly :-S

the thing is, i bought it used, the old user swapped the tuners, i don't know what model he put or what, but they're grovers, so i was expecting something better than that.. however, the strings keep going out of tune, could it be something else than the heads? i'm not so familiar with setups and everything right now, the strings are well strung and everything, but it just keeps detuning
It's usually the nut. Check out the setup thread in EG.
A badly cut nut with no graphite in it will definitely cause this. Sometimes it's a bad set of strings, or truss rod issues due to weather, or a few other things, but the nut is the first thing to check. If it was a used guitar, you might want to spend $40-50 on a good setup from a luthier, they can check everything out and get the action and tuning stability proper.
i've already tried the restringing, was better at first, but not anymore
the guitar is from 2006, the nut would make sense, maybe it's been played a lot, the nut could've been eroded, good idea =)
i just set the intonation, it sounds better but still goes out of tune frequently
I've got an SG special, and it never stays in tune. I've taken it to several luthiers, and there's not much to be done, or so they say. A lot of SG Specials are just lemons. Mine certainly is. Yours could be, too. Take it to a luthier and see what he (or she, although unlikely) says.