It might be worth checking you idle stablization valve. It's a silver cylinder normally with a VDO sticker on it, which is above the rocker cover on the right hand side. It should be buzzing when the engine is on, and it maybe worth taking it off and cleaning it, something like carb cleaner should do it, then rinse it with water, let dry and put back on. May work may not.
Cheers
Biz