you say you have a floyd, maybe the locks are a little loose.

maybe when you tie the strings the strings overlap and that causes slippage

maybe the springs holding the floyd (im pretty sure they have springs under them) are old and they get loose every time you tighten the strings

the strings are wound, and wound strings get a little loose when you tighten them so maybe you gotta give them time.

maybe theres something wrong with your nut

thats all i can think of.