Series 80 1HZ - puffs of black smoke

Hi,
I have a 1996 1HZ - 145,000 kms - on a long trip recently a mate of mine
who was following remarked that every couple of days the exhaust would emit a huge cloud of black smoke and then go back to normal.
Inside the car nothing unusual was noticeable and once I did actually see the cloud and it was large enough to obscure following traffic momentarily. Anyone have any ideas what might be the cause.
I have since had the injector pump timing checked and it was found to quite far out - 75 kw on the dyno as oppoed to 93 now. A small drip ws also found inside the pump and fixed. No long trip since so nothing to refer back to.
Would appreciate your comments,

regards,

Martin.