Vauxhall Cavalier SRi misfiring when cold; fine when warm

Hi
I've got a 1990 H Cavalier SRi which is misfiring when cold. It's almost like it's only firing on 3 cylinders (doesn't rev smoothly, jumps
occasionally) for the first 10 minutes but then very definitely clears up in an instant and runs fine.
It has only been behaving like this since I topped up the oil, so I could well have knocked something when I did that. I've had a good look about and can't see anything obvious; any ideas? What could cause it to go from lumpy and rough to normal in a second?
Cheers for any help,
Mark
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It sounds like it is running on three, therefore something which is likely to effect one cylinder, I have sometimes found that the injector connectors can become unclipped, so pull off each of those and spray with wd40 and refit, however the most likely is a plug lead, either loose or breaking down and leaking its spark. It may just be coincidence that this occurred after being under the bonnet, another vauxhall problem can be the servo pipe being cracked and drawing air, but this usually gives symptoms at all temperatures. Next thing to try will be to check and clean the distributor cap and rotor, then replace plugs and or leads, depending on which look the mankiest.
mrcheerful
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