Bmw e46 rough idle

[Glynr113]

Hi guys, anyone help please. Got e46 2.2 convertible (auto).Starts and runs fine but now and again at a stop it starts running on what I can only describe as 5 cylinders. Turn it off and restart and it clears till the next time (1 mile maybe 10 miles.Took it to an independent bm specialist who diagnosed oxygen sensors,then air leak which also meant replacing inlet manifold, breather and other pipes. Then tried swapping"control unit " ? from another bmw. Now he says could be head gasket.Cost me over £900 so far , now wants another £800 for head gasket but don't know if it will solve problem. Cars now becoming a cash cow. Any ideas anyone,thanks