Thanks for the ideas. I suspected glow plugs because of the rough starts, with clouds of smoke, when cold--coupled with the pesky GP light. Could the GPs be so sooted up that some have become ineffective? Maybe I've got two issues here: the relay and/or ignition switch, and the plugs themselves. There's a note with one of the maintenance actions that the light could be made to illuminate by tapping the relay, although I've not confirmed that myself. Still doesn't explain the hard starts, though.