Passat Syncro starting issue?

Hi I have a 98 Passat V6 Syncro and it has a strange randam starting issue?

The car runs great with no issues or fault codes stored etc but every now and again when I come to start it it fires OK but then cuts out and struggles to start i.e. turns over very slowly and queitly (Like a flat Battery but this is not the case) and to start it I will need to hold the throttle to the floor and keep turning it over until it fires, may take 2-3 attempts as soon as it has started it runs very rougth but when settled down runs great again with no issues???

Anyone know where to start looking, suspect it only does this when the car hasn't been used for a couple of days so could be time related but appears very randam????

Thanks for your help in advance?

Andy

i had this on mine and it turned out to be the senser to the crank i think...