2000 TD5 90 - Dying Fuel pump????

Hi all,
I need help / advice with my 2000 TD5 90.

The car has a Tunit box on it and when it gets to about 70mph the car
seems to judder, as though running out of diesel, after a while the
juddering dies down and the car will proceed to 80mph+.

I removed the Tunit and nothing seemed to be amiss so I quickly
thought it was the Tunit Box.

I went to Bell Auto Services and they re-mapped my ECU to try and
eliminate the Tunit Box...and they did, the re-mapping also produced
the same problem.

Reverting the car to stock ECU with NO Tunit Box I went for an
extended drive to see if I could feel the juddering. Whilst its not
nearly as apparent I am sure I can feel it.

So the question really is whats the problem?!? I have been told that
the fuel pumps are a common cause of failure on the TD5 and TD4 as
they loose the ability to supply fuel at a constant pressure.

Also, what should the top speed of a stock TD5 90 be? Yesterday, mine
seemed reluctant to go beyond 75mph, which I suspect is about 15mph -
20mph too slow.

I have recently serviced the vehicle (2000 miles ago), where all
filter where changed.

....long shot but is there a filter on the pump inside the tank that
could be blocked???


Thanks
Jon