[chedz]

I did something a bit daft while fitting these nuts that I noticed at the time and could be your problem.

The bolts I bought for the ball joints were a bit too long so I had to cut them down. I then fitted the nuts without grinding a bevel on the end of the bolts. This meant the tiny burs on the end of the bolt cut a tiny sliver of the nyloc plastic out as i did them up. I only noticed as one of them was left hanging like a little blue spiral. I suspect removing some of the plastic in this way would cause them to work loose over time.....

Could also be that they're too hot next to the manifolds and the plastic has melted a little or become hard???

Hope this helps
Graham