While that can happen, it shouldn't happen 4 or 5 times a year with
normal jib sheets (I'm assuming you're not using single braid SK75 or
something really slick and stiff like that). It sounds to me like you
know what you're doing and with apologies for repeating the obvious:
You might consider tightening your knots a bit more and leaving longer
tails. When tacking try to get at least a little tension on the jib
quickly after passing through the eye of the wind. Finally, wash your
sheets (or dip them in salt water) to get the slick factory coating off
the cover... Anyway, something odd is happening with your sheets, if
we all had that kind of failure rate none of us would use bowlines!

-- Tom.