My HD28 has a tendency to slightly sharpen the notes on the 2nd and 6th strings as I move up the fretboard. It's only those two strings, the remainder stay perfectly pitched. It also doesn't matter which strings I use (I'm currently using D'addario EXP12s), the effect is the same.   

Any ideas or advice on how to fix this ?, I use a Shubb capo and mostly non-standard tunings which means continually retuning two strings every time I capo above the third fret. More frustrating than anything, I've got used to it over the years but I'd be happier if it didn't drift as it can be a bit of a pain especially in a session when I'm using the capo between D and G in DADGAD

Does anyone else have this problem with this guitar or any other for that matter?

Chris