I have had the my current mt. bike for about 3 years. All Shimano Deore/XT parts. Lots of hard riding and miles. At least 900miles. I just replaced my chain. At the end of my last ride, it would not stay on the larger cogs. Found a link ready to break off. Also replaced the BB., was loose, figure to replace it also. Have only replaced the chain 3 times the life of the bike. The chain still has problems on the the larger 2 cogs, thought maybe needs a small adjustment, but notice the cassette is loose/woobles a bit. Have not taken it apart yet. Is there anything to look for?