Is that the one with the Lucas Epic injector pump?
Also probably the gutless 1868cc? rather than Fiat 1905cc, I had one in an 03 Doblo with similar symptoms many years ago, it had defied another garage and a diagnostic company before I bought it for £300.
I located the fault by first getting it to run perfectly from a five litre can on the wing, before tracing the air leak at the plastic connector at the fuel tank, as it relies on the Lucas pump to suck the fuel from the tank rather than the later version that has an electric pump to prime the injector pump it aggravates a simple problem.
Fiat 15 years ago wanted nearly £200 for the plastic flow and return pipes front to rear.
I bought some fuel pipe and clips from Motor Factors for £7 and sorted it.