I had a similar problem.Turns out the fuel pressure regulator was at fault. Look at the clear pipe that goes to the air box if you can see fuel in it there should not be.
You've said you've done the Fuel Distributor, but have you done the Warm Up Regulator?
The WUR ultimately controls the fuelling based on how hot the car is, if it has failed, it'll almost certainly cause you massive problems.
You can also try unplugging the 5th injector, these have been known to cause more trouble than they're worth, I run with mine unplugged on my quattro and it starts much better.
Hi James, is the 16v stuff still available? The thread's been quiet for a while, so I wasn't sure. I'm mainly after a 16v head or inlet manifold, and I'd like to know whether the cam and timing gear are included or already gone. I can pay postage or collect if that's easier. Cheers