You may have or have had two different problems.

When RPM suddenly drops to zero ... that's an indication of something
electrical like turning off the key.  The engine is still turning but
the tach says zero.  It could be in the Hall effect sender in the
distributor or a bad solder joint or bad connection at the ECU, for example.

Stumbling with the tach still indicating RPM could very easily be fuel
related.

Mike

Gregor Brandt wrote:
> '86 Westy
>
> I've written about this before, but am having troubles fixing it...
>
> My van would suddenly drop RPMS to 0, for an instant, before coming back
> to proper RPM, causing severe bucking.  After looking at the usual
> suspects I checked the fuel filter and it was filthy.  Changed both the
> plastic fuel filter pre-fuel pump and the high pressure filter post-fuel
> pump.  Van stopped bucking, but now it stumbles under load.  It never did
> it before.  It has to be fuel related, but I have no idea what it could
> be.   Any help is appreciated.
>
> Thanks,
> Gregor
>