'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