1500 shuts off

[bostonwhaler]

1998 1500 e

No boom on rear cylinder after new piston. Rings. cylinder etc.
Tested coil . stator ect plugged all back in changed rear plugs from 8 to 6 have boom
And will run for a couple of minutes but suddenly dies
Like hitting kill swich.
Hit starter same thing shuts off after a minute or 2
Any ideas?