It's my bandmate's guitar so I'll forward this message to him later.
We use regular D tuning (lo-hi D-G-C-F-A-D), and could this make any difference, because when the guitar is tuned perfect to D- and drop the E string to C, the other strings goes sharp. The E- string kept it's tune to C, perfect.
I stretched all the strings, like I do to my Ibanezes, and tightened the springs, and that helped a lot, but still there was a raise about 10-15 cents on the rest of the strings, when dropping the e string.
More thightening of springs maybe?