Citroen Berlingo engine cutting out

Developed a problem out of the blue when I was driving on the motorway and there was a slight thud and a feeling like the engine had misfired or was momentarily starved of fuel.
The car didn't stall at that time but this issue would randomly happen, seemingly more at higher revs.
Got the fuel and air filter changed.
All good for the first couple of hundred miles after that, then happened again, this time cutting out when I'm in traffic. Managed to restart but very unreliable and running badly after that.
Took it to a garage for diagnosis and they don't seem sure what the issue is. Told me to keep driving it for a while which seems crazy advice as I was lucky it made it to the garage! Would the injection seals going cause this issue?Advice please!