Vauxhall nova,intermittent spark?

Hi,
I have just bought a Nova for my sister in law but it has a fault.
Its a K reg 1.2 Injection with 59,000 miles.
It starts when it wants to start.When it starts you can drive it wherever you like but as soon as you turn it off it's a gamble whether it will start again.Sometimes it will start straight away but other times it may take a day before it will start.The coil,ignition module,distributor,rotar arm,cap,leads,plugs have all been changed and still it has the same problem !!
I've tried 3 other ignition modules and another dizzy and 2 other coils and still it wouldn't start.The problem is spark related and sometimes there is a spark but other times there isn't.The coil is getting voltage as it should.
Please help,i'm pulling my hair out !!!
Thanks