Check all the air pipes, turbo's hate un-metered air, it messes with there brain.

I recently had the same prob, irratik idle mostly at 300 revs then dying, everyone said air leek and after messing around for a month couldn't find any leaks. Then last weekend was changing the valve steam seals and noticed the rocker cover gasket had a chunk missing, whala problem solved.

Moral of story, turbo's hate un-metered air.