After about four days of normal operation, computer exhibited the "spontaneous reboot" symptom that had precipitated so many problems a few months ago when I tried to email myself some stuff this morning. This maybe-to-probably diagnoses the slot as the problem, since both RAM cards caused the same symptoms. Of course I wasn't going to hang around and see how bad it got this time; when I came home from work I promptly switched the same memory card into the neighbouring slot (the middle one of three, hitherto unused). We'll see how it goes.