1991 420SE starts and runs as long as starter is engaged??

[Mosser]

Hi All,

I have got a 1991 420SE (W126) right hand drive that stalled after starting one morning and has been the same ever since,

It will fire up instantly (no misfires) and run perfectly as long as i keep the ignition in the start position, as soon as i let go of the key the engine dies straight away, if i restart it, it starts perfectly and runs but again dies if i release the key!!

It has been to a MB dealer 3 times, the AA have had a look at it twice and the ECU has been replaced, fuel pump relay replaced (and another relay i cant remembe the name of!!) so far and neither has helped at all,

The AA got it running by shorting the fuel pump relay out and it runs fine then, but this cant be a proper solution and might cause other problems,

Can anyone offer any suggestions as to what to check??, I am pretty mechanically and electrically minded and have access to a multimeter and oscilloscope if needed

Cheers