I feel your pain, diagnosing a problem like this is usually a bit of trial and error. I would have expected your tuner to be giving you more info then they currently are though.

What do the plugs look like after it breaks down. I.e is one cylinder acting up or do all look the same? Igniter failure should result in one plug looking different.

I've seen injectors fail to work under high fuel pressure, although this has been well above 14psi boost pressure (Around 60psi fuel pressure). Are you using a raising rate pressure reg?
Earthing is one of the most common causes of retrofitted EFI system problems. Make sure you have proper earthing. You will be surprised what a few earth leads here and there will do, particularly if you have the battery relocated to the boot/tray.

That's my two cents anyway.