Did you try disconnecting the injectors one by one?
I know it sounds unlikely but your symptoms are similar to the one I had.
I also had injectors dropping out randomly. It was like the ECU was getting confused and dropping injectors to try to regain control of smooth running. It was not until I disconnected the injectors one by one that I found the Problem.
Speak to Jack in Autologic VAG. I went through it with him.
HTH
Fiachra.