2005 Citroen C5 hesitates in cruise constant speed

Would try replacing fuel filter first ,just in case .

Thanks for the reply! ... that has been done. also, the EGR tube cleaned.
The car is a little unusual in that it has a Hydrogen Generator fitted, (in 2011) which improves mileage, & emissions by burning the particulate. there has been no Eloys in the car for the last 3 years! (I regularly get 49mpg on short around town driving) & no problem passing MOT.
Citroen have suggested that it may be a blocked pipe, a blocked or faulty high-pressure pump at the tank end.
They also say that the fuel priming bulb perishes & leaks but that is all good.
I had the car checked by Citroen recently & from the time of booking it in, till the present, the car hasn't exhibited the fault again... so perhaps the thought of going into the garage frightened it!
for 150k miles she's still a fantastic car. with all the toys., The ride is great, & I love the Xenon Arc steering lights... fantastic in country lanes.