Try this quick procedure to check how the valve is operating
http://www.dummies.com/how-to/conten...pcv-valve.html
If it's working OK you could try running a bottle of injector cleaner through it and see if that helps at all
Looking at the easiest and cheapest options for you to try first, hopefully it will be something simple but a lot depends on how well it's been maintained over time. Have you changed the oil recently, if so are you using the correct grade in it ?