So for the last year my van has broken down quite a few times, ending up on the back of a tow truck home, try starting it the next day, all is fine and dandy!!

Each time it looks to be electrical as fuel is getting to the injectors.. It doesn't start on LPG either.
I've replaced the ignition amplifier, distrib cap and rot arm at recommendations from mechanics.. But they haven't been able to look/fix the problem as by the time I get over to them, it's starting fine.
Someone down the pub suggested it could be the immobiliser? I'm not too sure about that, as I'm getting ignition lights and the engine is cranking...
Please if anyone could at least point me in the right direction to how on earth I even starting trouble shooting this?
2.1 DJ injection - Petrol/LPG
Thanks ):)