Hey I've got a problem with the g string on my guitar going out of tune ridiculously fast. Anything that involves a couple of bends and it's gone, which often means it's out of tune mid way through a song. Any diagnosis/suggestions on how to fix this? Thanks
The G string is often a bit fussy as it's the string with the most tension on it, however what I've found to work for me is to tighten the tuner very slightly with a screwdriver, also if you haven't changed your strings for a while then that's certainly worth doing too.
The problem most likely is your nut. Either the string is too big for the slot, or there is some obstruction that causes your string to catch. If you have a 3x3 headstock the angle that G string takes from the nut over to the tuner is sharper, so it makes it more likely that it will give problems than the other strings. Widen the slot slightly and lubricate using graphite (pencil lead), chapstick, or commercially available product.
It's either sticking in the nut or you have it loose around the tuning peg. Keep a good tension on the string when putting it on and don't wrap so much.

Just give the string a good tug, tune, repeat until it stays. It should stay after you do this a few times.