The bike is about 10 years old with an average 30KM per week on it so roughly 15,000KM. It is a 3 * 10 speed Deore XT and it has performed great up until recently when my chain broke (bought a Park tool to make sure to check on chain wear).
I have an issue with my chain slipping while staying in the same cog. When my chain broke, I replaced the cassette and I have also replaced the two larger chainrings. The chain was shortened to the same number of links as the last chain. The larger of the new BBB chainrings has 2 more teeth than the original Shimano and the middle BBB chainring is the same size.
What happens is that if I use the smaller cogs on my cassette, the chain appears to slip almost as if it is skipping some of the teeth on the chain. This happens on all 3 front chainrings, but more with the middle and the larger one. The slipping is less likely 5 cogs in from the smallest (fastest) cog, and more frequent when on the smallest cog in the cassette. On 6th cog or larger I haven't had it happen. It is not a loud noise, and almost sounds a bit like a change of gears (without it moving left or right), however after today's ride I noticed some metallic particles that the chain is picking up, so it is definitely adversely affecting the cassette.
It happens while under very little pressure while going down a hill and it also happens when there is a bit of pressure on it. I haven't found it to be as much of an issue under heavy load. Unfortunately it is not persisting all the time. I can get 4-5KM without it having any issues and other times, I can't use 4th smallest cog even without it skipping every couple of rotations of my crank.
Also, a few weeks ago, I noticed that my derailleur bolts fastening it to the frame were very loose before this happened, and I have tightened it to ensure that it is consistent. I was amazed that everything worked as well as it did with one of the bolts being very loose.
I have adjusted the gear lever so the derailleur is perfectly aligned with the cassette cogs.
Any suggestions on what this could be? My guess is the derailleur might be worn or the springs might have weakened. Could it be a link in the chain?