Volvo: Check Engine Light, spark plug wires, pcv valve

Question
Hi, my 1997 volvo 850glt (100k miles) has had an annoying check engine light that refuses to stay off. First it was the secondary air injection which I replaced all components on that line. Now it's saying random missfire on cylinders 1-5. I've replaced the distributor cap, rotor cap, coil, spark plug wires, and plugs themselves. What could be causing this? One person suggested a vacume leak off the intake manifold but where should I be looking? Could a clogged PCV valve be to blame?

Answer
The PCV valve could be suspect, but I would expect a rough-running engine (or noticably different, at least) to accompany.  I'm not exactly sure 'what' is telling you about the misfire, but that might be a timing issue.  Have you ever had work done to the camshaft or replaced timing belt?  Does it feel like it's off-balance, vibrating more than it should?  Any noises?  Did the light come on before or after replacing ignition components?
JD