Are the LA210 printer schematics/tech info available anywhere? 
Probably not a lot of help for this fault, but the digital side is pretty
similar to the LA100.
 I have one that will print a few lines, then go through some kind
 of reset (printing, lights go off, come back on, loss of text, repeat)
 Self-test print fails after a few lines in like manner. Prints a few
 lines, resets, and stops printing. 
This sounds very much like a PSU problem. I would start by seeing if
any/all of the PSU outputs drop when it's doing a self-test. It may be
something as simple as a dried-up capacitor in the PSU.
-tony