What about carrying out a volt drop test on the earth that the ECU uses for controlling the injectors?

Do you lose comms when in a non start condition?

Will it keep doing this 5 second start/stop all day or do you have to leave it for a time between starts?

Is the fuel pressure pure fuel and not aerated fuel, have you done a quantity (flow) check?

Have you tried running it off a separate supply?

Did it start doing this over night or is there a history to this vehicle?

Has it got the right oil?

Regards Mark