Lumpy idle on E34 525 TDS when warm

On cold start-up it seems to run smoothly, but when warm or after a few minutes, idle seems to be lumpy, as is misfiring on 1 cylinder - it still pulls
away smoothly when warm / hot, apparently firing on all cylinders, so I don't think its e.g a weak compression cylinder (cracked piston ring or leaky valve) its just when warm / hot that it runs as if one cylinder not firing.
The car has been little used for some months, so this might be a clue.
Any suggestions as to what to look for ?
Would it be worth putting some injector cleaner through, or are these symptoms a bit more sinister, like the injector pump about to die, although I would guess it would affect all cylinders if this were the case ?
Thanks,
Nick
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go to this site and ask there.... they specailize in e34 http://www.bimmer.info/forum/forumdisplay.php?f 

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