i recently put a set of not even slinkies on my mexican fat strat and it wont stay in tune (tuned to C) for more than a few minutes. ive filed the slots in the nut to accomodate the girth of the strings and have tightened the claw screws, but now when tuning, by the time i get to the 4th string, the 6th is a half step flat. whats going on???