I have a similar problem, but got 1 new front sensor (hellish expensive for what it is!), and no different, even after swapping around, and the magnetism seems to be the same as the old ones so I guess the front sensors are ok. not tried the rear ones yet, & assume they are also mega bucks, so will live with it for a bit longer.
Havnt tried the shim trick on the rears yet (if they have shims)
It doesnt do it all of the time & only when slowing to a stop, seems more frequent if also turning at the same time, but that may just be coincidence.
BTW chris, did u sell your injectors?
Andy