My thought is Ignition Switch, if you say all other starting/electrical systems are OK and it happens hot or cold intermittently.
May be someone else can chime in........

__________________
FRED

Daily Driver: 98 E300TD 199K
Hobby Car: 69 Austin Mini
Past Diesels: 84 300SD, 312K
87 300SDL, 251K
94 Chev. K-1500 6.5Ltr.TD, 373K