Good morning. I am a newbie but as I want to post a problem thought I'd leap straight in here. I think I may be spending a lot of time here

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Have owned a 2001 2.2 petrol estate for about 6 months. For the last 2 or 3 I've had on-going issues mainly varying degrees of misfiring.
On first inspection found loads of oil in plug holes so replaced cam cover gasket, dis pack and plugs -made no difference in fact ran worse.
Then I cleaned all the breather tubes. Made no difference. Running by this time had become very erratic and down on power.
Decided to take it to the garage and let the trained professionals loose. They changed the MAF. I took it for an 80 mile spin the next day and seemed to be fixed. Took it out 2 days later and after about 5 miles started mis-firing again. And similarly on subsequent trips.
I blanked off the EGR but this has made things no better.
Car starts fine, first time, whether hot or cold, and drives as it should for about 10 minutes/5 miles and then starts misfiring and well down on power.
Any help would be appreciated as funds are running low, as is patience, and I need a car I can rely on to go when I need it.