Several people here were kind enough to provide suggestions when I had problems with "tearing" threads. While some of them improved the threads, the basic problem remained - after a couple of passes the threads would get a jagged, torn appearance. Nothing I tried changed this.
I finally started examining the thread after each pass with a magnifier: First cut - no problem, even & smooth. Second cut - I thought my eyes had gone weird... the width of the (flat) crest was varying.
The lead screw looked OK, so I disassembled the apron and voila! The half nuts were badly damaged. The threads were mostly torn away, and the entire casting (they look like a die cast part) had many small cracks. I believe this was allowing the nuts to move after engagement, occasionally changing the position of the cutting tool relative to the previous pass.
Of course these are the challenges of working with a 60 year old lathe (10" Atlas) that came out of a shed on a friend's barn :-) I ordered new half nuts from Clausing and installed them.
I threaded a test piece of 12L14 today. The "tearing" is no longer evident, and the pitch is constant.
The flanks of the thread are rough, it looks like I am getting a bit of chatter, I'll play with my setup and tool geometry per the advice previously provided to try for a cleaner thread.
Thanks for the advice guys, you convinced me that there was more at play than my technique.
Rob