It's the boring option, but why not take it to a Citroen or Peugeot dealer and let them take a look?
If that's not sensible (too far away, perhaps, to drive a car that's playing up a little) then try a good independent.
1.6 litre petrol Citroens and Peugeots are not outrageously complex under the bonnet.
It could be just a coolant temperature sensor or some other simple little component that is refusing to play ball.
Don't keep guessing, you'll be spending £££££ on fuel and getting nowhere fast (or slowly, even).
Lots on here seem to argue a case for the EGR valves to get gummed up.
You might try a can of Forte Gas Treatment and a few prescriptive 'purposeful drives'.
Of course, you must NEVER exceed the posted speed limit. That would be very very very bad indeed

!!!!!!!!!!!!!!!! And none of us has ever exceeded the speed limit. Ever
But you could hang on to 2ng gear until 60 odd mph or so for a couple of minutes.