Hi, long time lurker, but here's my first post..
This is a common problem with rubber separating on crank pulleys. It's normally an easy fix once caught, but if left too long it will knacker the nose of the crank. Normally there is a woodruff key and it will vibrate causign wear in the keyway on the crank. This is not the end of the world though. You can get a new woodruff key and fill it into the slot/keyway with chemical metal then file to fit. This works for a long time. My old MX5 had the problem and I fixed it like this back in 2006 with about 250k on the clock. It's current owner ( a friend) is still going strong with it and it's got another 100k on the clock. You can normally see the crank wobbling when it get's bad, but it starts with a bit of noise and bad vibes.
Another possibility. On my old TD4 Freelander, the injectors were shot so I got all 4 reconned and balanced. I then bought a tool to reseat the injector seats in the head. If you get leaky washers, you will get residue on the seats, and no matter how many times you change the washers, you will always get leaks and poor running. The tool is about £15 on ebay and is pretty universal for most cars if you buy a kit.
Hope this helps.
oops, I see I need to update my car now I have a 147 JTD and not the Bloody Freelander anymore, or the 3 that came after it...