Does it do this at idle? Do you have to raise the idle speed to keep it running?
Have you pulled the plugs out to see if they are all burning the same?
As you can see a problem like this is very hard to figure out over the internet. Too bad most shops have morons for mechanics, they are parts replacers not mechanics, so if something out of the ordinary comes up they don't have a clue. A lot of the shops around here have never even sold a XX let alone worked on one.