As I recall, the "spec" is 18 inches clearance from AC and ballasts in lighting. I could be off as I haven't seen the actual doc recently. What you're describing sounds to me like crosstalk, though. This happens when the installer untwisted the pairs too much (half inch MAX) or installed them into older patch panels that forced this.
What I have seen a lot of is cheap "Cat5" that doesn't meet the spec. Cat5 should have three twists per inch; I've seen a lot of stuff in old installs have much less (it's really Cat3 but is labeled Cat5). This means you're getting crosstalk along the runs themselves; usually this is alleviated by the time it gets to the end but not always.
Anyhow, there are a lot of things that can affect this. You really have to start at the demarc and follow the runs from there, narrowing down each possible cause of the issue. This can be time consuming, especially if you don't have a really good tester (as in more than just continuity). I use a Fluke OneTouch 10/100 myself but those are pricey, even for an older one like mine that doesn't bother with fiber. Still, they can be invaluable in cases like this.