The car is a 2001 51-plate TDDi estate, ex-taxi with 125,000 miles.
We only bought it a few weeks ago, and being an ex-taxi it's a bit rough around the edges but mechanically it has been fine up to now...
I was driving it yesterday for about an hour, and thought that when it was cold the engine sounded a tiny bit rattly under throttle, but not enough to worry me really, and apart from that it was fine.
Then I was going uphill, applying quite a lot of throttle, and all of a sudden the car had a very sharp "hiccup". It was then fine for 5 minutes, and again did it, and wouldn't stop. I pulled up at the side of the road and noticed that if I put the accelerator on the floor, the engine will start rising but then randomly drop revs to idle for a moment - so it was pretty much impossible to accelerate smoothly. Also, while this was happening the exhaust was belching out tonnes of white-ish smoke.
I looked under the bonnet and there wasn't enough oil (everything else looked fine), so I got some oil and filled it up, and then drove off... And for 10 minutes it was fine again. And then all of a sudden it started the same thing and my speed dropped from 70 to 30. I parked up (luckily there was a parking area just there on the dual carriageway) for half an hour and then drove home again (with AA truck following me) and it was fine all the way home (about 15 minutes).
So any ideas what it could be? It doesn't look like any oil/water is mixing. And it's not overheating.