misfiring vectra?

hello my vauxhall vectra has suddenly started missfiring. it starts in the morning and runs fine but after about 25mlies it starts miss running rough. after i have been at work it starts ok again and then afer about the same amount of miles it starts to run rough again. if i start it again when im home it runs rough. it seems that once it has been left more than 6 hours it is fine again. i have replaced the spark plugs. any ideas . thank you