mines started doing this recently,since funilly enough since having had the spark plugs renewed at big cam belt service! far too much of a coincidence!! thinking plug leads perhaps as my rockers were also leaking,had new rockers fitted in march by my garage as they said theyd warped,then when he did the plugs 3 weeks ago said youve got oil in your plug wells drivers side,so i said i should hope not money i paid you in march to do them!!.hed forgoten car had even been in for this job!! seen the price of new leads about £154.00 aftermarket,may go for good genuine vaux/secondnand!