As Sonoronos suggests, check the wiring/coil. Espeically if your car is in a humid environment. When looking for loose wires, open the hood in the dark, jumping spark will be more apparent.

Noisy valve train is another possiblity, either from excessive oil drainback from sitting for so long, (I don't think a week is too long) or a sticky/dirty part in the valve train...as was also suggested these may have a chronic valve problem I'm not aware of, embedded in the forum somewhere. Don't envy you with the forum search function, sometimes sifting through all the BS to find useful info is more difficult than bringing the car to a shop. Let us all know what you find out.