Minor correction, the LAB-400 is fully automatic as well.
I should have had a look at the Lab 400 service manual, which was on my PC hard drive, before answering.There are other things like the plastic on the pivot can crack causing the rod to slip inside the cam that moves the arm. It's not a big deal. Nothing a drop of super glue can't fix.I'm not sure of any specific common issues with the LAB 400 ... but it's very common for the rubber isolators / spring boots on the PL-600 to be deteriorated. Also the tonearm belt typically needs replacing. Something to check if you are looking at one.