not experienced this before, mostly my guitars will go flat from bending or playing hard and I have to tune back up.

been using my boss tu-3 to check and every time I play for about 10 minutes, my guitar goes sharp, on all strings.

it's not the neck position, I keep it neutral and have tried putting pressure on it both ways to see the pitch change but when it returns to neutral it stays in tune.

I can only assume it's something like the tension of the truss rod increasing as the guitar neck warms up with playing, pulling on the strings more, but fack knows.

so basically I have to tune, play a song, finish it - check my tuning, it's always sharp, so I'll stretch the strings out by holding at first fret and pulling outwards till they stretch, then tuning back up.

play another song or 2 and then check tuning - it's sharp again! repeat, bloody boring.

gibson deluxe tuners on a faded 2005 sg special, once it's in tune it does stay in tune (for a while) and it's not slipping at the nut at all.

again it's definitely not the neck being forced at an angle or lean etc, it's just neutral playing position and the same position I tune in.

any ideas/experiences appreciated!
sounds like an intonation issue to me. either that or your nut is bad and the strings are getting hung up in it
yeah i always stretch strings in when i first put them on and then keep doing it until it stays in tune.

using rotasound yellows 10-46 as they came brand new on the guitar, they are ok but want slinky regular or skinny/heavies for sabbath c# tuning.

i reckon your right with the intonation thing, the bridge/saddles are not that solid and maybe palm muting is moving them back slightly. the screws that adjust the bridge height are flimsy and want to be thicker really. might upgrade the bridge, noticed a lot of people have upgraded to schaller roller bridges on sg's and have fine tuners on the tailpiece
check the nut inserts on the bridge to make sure they are not wiggling around in the body it is going sharp so the bridge is getting pulled in towards the neck. the neck and truss rod dont move that well without finishing it if one was to adjust the truss rod you have to understand how each piece works as one. meaning you do not want to crank the truss rod one way or another and you make adjustments slowly and you have to finish it while doing it that is last resort when all else checks out. even if the nut was completely loose the strings hold it it place. I am setting up a bass right now and i have a nail with slots cut in it and it stays in tune not prob. if you have thinking problems like me sometimes just remember if it is sharp the bridge saddles and nut are to close together flat they are to far apart long winded but those sg bridge bolt inserts can get loose on softer wooded bodies. I made three guitars before all the technical stuff stuck in my brain.
if they are loose or you just want to make complete certain just pull them and get a 2 part slow set epoxy and reset them.
JUST as a note I am trying to get what intonation is just the half way point of whatever scale the instrument is at the 12th fret ,meaning tune all the open strings and then hold down the strings one at a time it should be the same note just on octave higher. this will change on some new string changes change in brand or gauge. they usually run sharp also but that is not the problem you have.