A good diag tech should be able to trace the misfire without chucking loads of parts on it ! A smoke test, ( known as leak test) is worth carrying out, this will trace any air leaks in the manifold system. There is a rubber breather pipe low down that rots and causes missfire & rough running problems & is hidden near rear of head. A smoke leak test will find this fault in couple of mins. Good diag garages use these units.

I trust the engine is running on the correct grade of oil ? if the oil grade is wrong or has a poor quality oil filter the oil pressure can run too high causing the tappets to jack up causing misfire! I have come across this many a time where they have been topped up with the wrong oil!, but check for airl leaks first & an emmissions check with someone who understands the readings will know whats wrong.

Don't use WD40, any silicon based product can have a detrimental effect on the operation of the HEGO sensor's ( oxygen sensor) as its a contaminant.

Find a garage that has a "leak tester/smoke -pro" these will sucesfully test manifolds and injectors seals etc & breather pipes for leaks very efficently.

I have used leak detectors for some years & how garages can operate without I really don't understand as they save hours of time and shed loads of money in parts!