[minicoop900]

chains usually dont stretch, they made them pretty well, your more likely to just be off a notch or two just from someone not adjusting it right or tensioner is a possibility too, if it goes weak, it could make the chain skip a tooth or something.

may be pcv valve, easiest way to check for vaccum leaks is to take a propane tortch, open it just enough to get a little flow, and stick it around areas where it might be, and listen to the idle for changes. if it changes, the area your in is where the leak is.

and i hope im helping you, lemme know how it turns out.