So paid $30 for 4 new servos, reflowed the QFN's solder joints, & made a servo exerciser to thrash it all night.  Naturally, it would never fail on the bench, but the new servos alone stood a good chance of reviving it.  The last order of standard servos was 2001.  They were $6 Tower Hobby branded, but shipping was much lower.   Those hobbyking servos are now $5, but shipping has followed inflation. 

Signs started pointing to a glitching battery connector crashing the microcontroller on the servo.  The mane CPU also had glitches, but didn't lock up.  The servo may just lock up under certain power glitches.

After cleaning all the battery connectors, the Trackstar was revived & didn't crash again for 400 miles. So it's a lot more sensitive to electrical noise than a brushed servo. It must run at a much higher clockspeed.