I've got a funny feeling they do switch off the injector if the misfiring is bad enough to risk catalyst damage. IIRC this is reset if you stop and restart the engine. The problem is of course that the misfire could be ignition, injection or mechanical (i.e. sticky valve as mentioned) but alway ends up with the injector switched off.
I replaced an injector in a Pug 106 once but I think it was a dodgy plug that was causing the misfire as I had to put a set in before it would run properly .
Have you checked the injectors electrically, I use an inductance meter now rather than a resistance test as it shows up the faults much better (I started a thread on this recently, look in the archives).
HTH, Shaun