2000 Defender TD5 wiring fault

Hi All, I'm new to this forum and I am hoping someone can assist me in solving a very strange problem with my 2000 Defender TD5. I bought the vehicle as a non goer and fitted a reconditioned engine and replaced the Crank sensor. It starts easily but as soon as anyone sits on the drivers seat the engine cuts out. Sounds weird but true. I have tested all the wiring into the computer for continuity, shorts, and earth leakage but cannot find any faults. If you push or pull on the computer or associated wiring everything is fine and the engine keeps going. Sit on the drivers seat and it stops. I think it has to be something wrong in the wiring somewhere but I cannot find it. Plug in the Hawkeye and everything is OK. Sit on the seat and it looses the data connection. When the engine plays up the "check engine" and "glo-plug" light flash on & off.
Anyone got any ideas or seen the problem before.