Problems like the one described by Maktheknife happen with some frequency on certain Rega arms. I've seen several posts like his over the years.
The fact that yours is mistracking on only the most challenging LP's may simply indicate that the wire or insulation is out of position by only a small amount, so that the pressure it's applying is small. This might indeed cause audible mistracking on only the most challenging passages. If you read his post again you'll see that's just the symptom he had, "All of a sudden SOME of my ... LP's began skipping".
It can't hurt to check the routing of your tonearm wire and its condition. While you're at it, dismount and remount the cartridge from scratch - just to make sure everything is still set up properly.
The fact that yours is mistracking on only the most challenging LP's may simply indicate that the wire or insulation is out of position by only a small amount, so that the pressure it's applying is small. This might indeed cause audible mistracking on only the most challenging passages. If you read his post again you'll see that's just the symptom he had, "All of a sudden SOME of my ... LP's began skipping".
It can't hurt to check the routing of your tonearm wire and its condition. While you're at it, dismount and remount the cartridge from scratch - just to make sure everything is still set up properly.