4.0L v8 starting rough

Hi all, my 4l v8 p38 has just started to run rough on start up.
when started from cold it sounds like its not running on all 8
(even when you rev it) but only for about 30 seconds then its running on all 8.
she was running perfectley since i changed all plugs and leads about 8 weeks ago.

Delta351

Damp on the leads or caps? Leads lying on top of each other?
Try starting it up after dark, stick your head under the bonnet and see if you can see any sparking, keep your fingers away!
Try and separate the leads as much as possible. A cable tie in a figure of eight (loosely) round two touching leads can be enough. It's an idea I nicked from watching a gardener staking tree saplings and has been successful for me a few times.

Thank you for replying dogsbody ive just found the fault!
Its the battery it only had 11.8 volt i just charged it for 40mins its now at 14.7 now it starts perfectley
firing on all 8 straght after start up.
regards delta351