Alan Marshall, the luthier of Northworthy guitars let me into a secret a few weeks back. If I'd known it about 15 years ago it would have saved me getting rid of a perfect Martin D35 that wouldn't stay in tune. That bastard guitar made look an idiot in front of an audience just once too often.

Alan told me this, 95% of tuning problems can be sorted out with two things

1) get a nut where the size of the slots has been worked out with micrometer - dedicated to the strings you use.

2) Get some one who REALLY knows what they're doing to choose the pegs and make the holes the right size - so that the strings seat right.

I'm sure people will disagree, but he's a really good guitar maker and he knows a lot, and sees a lot of guitars. Worth a try before you chuck out that Martin.