Hope this is some help. I'm retired now but after a career in Technical Support one does develop a certain amount of "nouse".

Been there done that with intermittent HDi power loss which was so annoying that I almost phoned the scrap-man. Typically a 306 HDi will cut out for about a second on a five mile drive. If the accelerator was left alone (foot off) during the "hiccup" the power would return. The impression given was that fuel starvation was occuring - it probably was but not in the way that one might expect! Computers all work in the same way of course and RUBBISH IN = RUBBISH OUT so for a while the Throttle Position Sensor became the chief suspect. Eventually it was noticed that "pedalling" the accelerator during a "hiccup" did make a slight change but how could the potentiometer track be intermittent in all positions?

Next the High Pressure Pump became suspect as perhaps it was worn and not making enough pressure? I had similar with a petrol powered Audi 100 and if there is insufficient pressure in the fuel injection system the injectors won't spray - SIMPLE! (Long story though but four or five previous owners and their garages were all beaten by the beast) Now those HDi High Pressure Pumps cost MORE than an old 306 is worth so one might have a problem. Some places ask about £700 for such a pump although £450 to £500 is now the ball-park figure for a rebuilt and guaranteed pump. This is still a lot of money and of course it is a cam-belt job. A quart of two-stroke oil was added to the tank to see whether the extra oiliness would change anything but no great change occurred although the car did seem to go slightly better.

Remembering that the VW Audi 1.9 common rail turbo diesel can have injector wiring problems with vibration fracturing the conductors inside the plastic, caused me to look at the wiring to see whether mice had gnawed it etc. Nothing was obvious at first although I could tell that someone had been messing with the injector wiring looms as the way that they had been taped-up was not OEM.

It was then decided that the ECU connectors needed to be unplugged and examined. Although the best light of the day was now fading it was still obvious that all was not well. Some of the ECU pins were tarnished and one or two had verdigris growing on them. VOILA! The problem was how to clean them especially as this type of connector is only rated for a few dozen matings. "Rocket" maintenance fluid was sprayed on both male and female contacts and the connectors were then plugged back together (often just a "re-seat" will cure bad connections) I was lucky as on my ECU the pins had not broken or corroded away which is a problem that some people have.

After this doctoring the results were like having a new car as it would now take pedal-to-the-metal whilst going uphill. No more "hiccups" or power loss which was a real boon. I actually know someone who squandered £3,500 on his VW Caddy at a Main Dealer and there was NO WAY that I was going to go down that route. What worked for me may not work for you but just remember to use the "Softly softly catch the monkey" approach. Rushing in with OBD diagnostics will be no use at all if there are corroded pins everywhere. Good luck!