1996 Vauxhall Astra warm weather running problem

I live in portugal and as the weather has warmed up to the high 20s then my car has started running poorly under acceleration after 20000 rpm, feels like fuel blockage or missing a spark from time to time and stutters . Had a few days of colder weather under 20 degrees and car ran very smoothly again, i have a feeling its probably one of the sensors somewhere have a fault but im hopeless with the electronic side of cars so unsure what its likely to be, suspect air flow meter or switch but not sure. Any help or advice on this subject would be greatly appreciated. thanks.