1994 k1500 with 6.5l diesel.

Following have happend to me three times within the last 2-3 days.
Engine starts fine and run great.
I put in reverse gear and go backwards 5-15 meters and engine stops as I should turn off the key. It starts again, but I have to crank it a lot in 3-4 sessions with a break in between. It hesitate when starting up and then runs OK
I have changed PMD/FSD wich is relocated on a cooler to above left fender. Third stop was with the new PMD.
It has not happend when in Drive and go forward.
I can also go fwd in drive a few meters and then reverse and go backwards without shut down ??

Have in my mind to connect a small lamp in parallell with Fuel Shut off Solenoid to check 12V.

Question is what in connection with reverse gear can cause shut off fuel ??

Regards
Borjan