this happened on an Astra I used to have(running on 3 cylinders for about 5 mins from cold in the mornings etc) and all I did was to buy some flusher for the engine once and flusher for the hydraulic lifters from Halfords(may have been WYMMS products) I think replaced the filter and I ONLY used genuine Vauxhall oil after that and it started to run on 3 cylinders for a couple of days before it went back to normal and is still ok NOW as I know peep who have it now!

ps I was told also that the head would have to come off to fix the problem BUT the above worked on mine!