Cristof -

I had a problem similar with my strat... funny thread too but that's another story (g string at the nut for those of you who replied on that one :D )

Have you changed your strings recently? What happened to me is that I tried a heavy gauge string and it didn't sit right in the nut. Once I changed to .10's it's sounding better than it did with the .09's. Funny how a small change can make a big difference.

What about the bridge? have any changes been made there besides the string height?

If none of these are the case, best to take it to a guitar tech.... could just use a good setup