Yea they get really out of place, sometimes so bad that two strings touch each other so I'm always picking at my racket. Might get it re-strung, how much does it cost on average? Day light robbery this city.
For a tennis racquet with decent string, I would pay about £15-18. Maybe a tab more. I paid £15 for my squash racquet last time. If your strings move about a lot then they are probably quite loose so if you get them tighter then you might lose a bit of power from your game. Up to you though.