Had a 16 valve one with similar symptoms. Tried 2 coils in it believing the first one had to be faulty. Eventually checked current draw to injectors. No 2 injector current was dropping out intermittently. I would have sworn it was ignition related as it only happened under load and was very random. The codes did not always pick the cylinder either generally random miss or sometimes cylinder No 3.
Hope this helps.
Fiachra