It could just be the nature of the yarn.... I agree with the suggestion that a smaller needle size might work. Smaller needles will make the fabric tighter, and that will make the sock stronger and last longer (nothing lik espending time knitting a sock, and having it wear through before you've even put down your needles!).
Something else that helps me keep stitches tight is to pull the yarn extra snug after I insert the needle into the next stitch.... I put the needle in, pull snug, then wrap around and finish the stitch. Another trick that works for me is to knit the first stitch on the needle through the back loop... the twists the stitch, making it tighter (it does look a little wonky to do it this way to the experienced eye, though!)