[78-L] a shuddering stylus

[Malcolm Rockwell]

Has anyone else besides me noticed a little shudder of their tone arm 
once a revolution on some pressings? It's especially prevalent at the 
outside edge of indy 1950s issues, but not limited to them and can cause 
the stylus to jump out of the groove in some instances. In fact I 
recently received an RCA-Victor record with this problem. It seems to 
disappear the closer to the center of the disc the groove is.
I believe it's due to an unbalanced (or badly lubricated) lead screw, or 
possibly a bad bearing at the outer end of the lead screw on the disc 
cutter.
Any other ideas?
Mal