Rpm dropout and ECC-IV code 57

[rtg]

Last week the engine started misbehaving - it intermittently dips in RPM. It runs really smooth and then rpm drops to near zero. I can then hear the injector piezo's buzzing, big inrush of air through the AFM's and then engine recovers with a bit of overshoot. Has anyone come across this type of fault?

My initial thoughts are electrical, so I've plugged in the Gunsons code reader and it's giving 11(OK) for test 1 & 2 apart from on test 3, where it reports 50(european ecu) and then displays 57(Intermittent in Neutral/Drive Switch (NDS) circuit). To the best of my knowledge I don't have one of those. Any ideas what 57 actual is?

The plan is to perform a vacuum test (when I've figured out how to..) compression test and then use a scope to check all sensors for noise & faults which the reader cannot detect. Also, any chance this could be from the distributor module losing timing?

Any ideas welcome!

[rtg]

Yes, engine speed slows (nearly stalls) - rpm indicator then tracks the change.

[rtg]

Thanks for the info - I've recently cleaned out the ISCV as it was sticking, so should be ok. The problem's like a cylinder (or burst of cylinders) intermittently failing to fire. It always happens at idle speed, but more frequently when hot. Under hard acceleration & normal cruising speeds it's fine.
I'll see where i get to this weekend but would rather solve this myself - that way I've learnt something new!

[rtg]

A possibility - it does lose all power for a fraction of a second and then recovers. The wiring on the s could well do with replacing, but it's a job for the winter. I've got a 4 channel scope so will be working around the electrics this weekend.