Hi, I have Iveco Daily 2.3HPI, cuts out after 50 odd miles?

After doing 50 odd miles, it cuts out with the engine management lighjt referring to an injector problem. Had the electronic diagnosis done but it does not show any faults. Once it cuts out, if i wait for ten minutes or so it starts up aghgain. Also cuts out sometimes if I go try to go over 70mph. Its ex parcel force van and is capped at 78mph. Any suggestions ?. (AA thought it could be the low pressure fuel pump ?)