1.6 HDi 2004 307sw

I can't believe this bloody car. I have only had it a couple of months, it
is the most I have ever spent on a car and I have already had more problems
than 20 years of driving crap old cars has produced.

Today's problem is (I will ignore the fact that the radiator fan switches on
and will not turn off again until the battery has been run flat on every
journey. That is another post in a minute), The car seems to run fine when I
am going through the gears and speeding up but I have noticed that when I am
cruising along in 5th at about 2000 revs the car will start to shudder a
lot. So bad in fact it is similar to only running on 2 pots in a petrol car.
When I put my foot down again it seems to go OK. There are no problems when
slowing down either. It seems to only be when going at a steady speed. At
other speeds it also seems to be going along fine and suddenly loose a
little power. It is quite noticeable to me the driver but probably not to
the passengers. It doesn't then pick the power up again, I then have to make
a slight depression on the pedal to maintain the same power.

The second power problem that I have mentioned has been like that from the
start. The first but main problem, the juddering, I have only noticed
recently. About the same time that I noticed this I have had an anti
pollution fault appear and the engine warning light come on. It stayed on
for 2 days but has now turned off and hasn't appeared for over a week even
when the power problems occur.

What could this be? Has anybody else had the same problem and solved it?
How? Could the lambarda sensor cause this problem and is it likely to be
that?
Cheers