Here's the $64,000 question though, which nobody seems to be asking - when you look at the negatives themselves, do you see anything abnormal in them, or are you relying on the scanner to tell you what's normal and abnormal? I'm not saying you shouldn't apply greater rigor to your fixing regime (and start by replacing your fixer more often - chemicals are cheap by comparison to the images captured on the film, which may be irreplaceable), but have you looked into what could be going wrong with your scanner? Any deeper discussion of scanning issues is better handled over at, the APUG sister site. There are some really skilled folks on that site who could provide you with immediate diagnosis if there is any scanner issue.