Chain slipping on 10 speed Deore XT with new cassette, new chain and new chainrings 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 have 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 (withouth it moving left or right), however after today's ride I noticed some metalic particles that the chain is picking up so it is definately adversely affecting the casette.
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. Unfortunatley 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 deraileur 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 deraileur is perfectly aligned with the cassette cogs.
Any suggestions on what this could be?
My guess is the deraileur might be worn or the springs might have weakened. Could it be a link in the chain?
Are you really sure it’s not jumping to the next cog? Sounds most likely with new parts.
I’d check that:

the cassette is correctly installed and tightened (it needs 40Nm tightening torque which is a lot)
that all links of the chain move freely, especially where it’s closed (I hope you’ve used a master link or a dedicated closing pin). If you’ve used a closing pin, make sure it’s flush. If it’s a master link, make sure it’s fully closed.
that the rear derailleur or derailleur hanger is straight and not bent.

Hang the bicycle somewhere (or put it upside-down), pedal with your hands and closely observe the chain as it moves. Check that everything shifts smoothly and precisely.
If the chain is actually skipping under load it should be much worse on the smallest chainring.
Regarding:

Also, a few weeks ago, I noticed that my deraileur 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 assume you mean the derailleur hanger (which is usually fastened with two bolts)? It’s mainly held on by the rear wheel’s quick release axle which firmly presses it to the frame. Usually you can actually see the derailleur hanger move slightly when you tighten the quick release.

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

I hope it’s not too short now. Make sure the derailleur is not fully extended when shifting to the biggest cog while on the big chainring.