Sure it wasn't Darren rather than Duncan?
Lookig through the setup's I've sold I did sell one to Darren in Telford and he didn't screw the amp to the bodywork (which is how I do it), he just wedged it behind the rear speaker panel with a load of Polystyrene and bubble wrap, so I did tell him it would probably overheat, which is maybe whats happening. Th Alpine V12 has a thermal cutout and can get quite warm, so if it's not installed correctly it won't get any airflow round it to allow the heat sinks to do their job and will thus shutdown.
It can't really be a faulty amp as I've never had one fail, and generally if they fail it won't be intermittant.
When the amp shuts down, if you then turn the volume down does it take a while for the amp to come back on again or does it come back immediately?
If it takes a while then this would point to a thermal cutoff, as obviouly the amp won't come back to life until it has cooled down sufficiently.
If this is the case then you need to whip off the rear speaker panel, take out any padding that's in there and screw the amp to the inner skin as I recommended originally.
Let me know if you need instructions on removing the interior trim as it's not totally straightforward.