Intermittant total loss of power / engine revs

I have a problem with my 400 SDi. The car starts and pulls away fine but then engine revs will suddenly drop when you are driving along. If you put her in neutral and press the throttle she revs up and will pull away but revs sometimes drop right off again and hence road speed also drops. I have had a general look around the engine bay and cannot see any chaffed wires or loose connections. I have disconnected the MAF but that makes no difference at all. This happens at all road speeds up to 60mph, but sometimes you can take her out and all is normal and she drives beautifully. My wife says that this morning it was really bad and each time the revs dropped she would ease off the throttle and put her foot down again; the revs would pick up but immediately die again (but not cut out). This evening I took the car out and it ran perfectly.

Please can anyone point me in the right direction?

I was going to do the paper clip across terminals 4 and 15 of the OBD connector, but I can't find the connector - so can anyone point me in the right direction here?

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