Nick and Tim, thank-you for your responses. Obviously (Nick) I have some in-depth reading top do, and Tim thx for mentioning the Slave Select line for the SD Card. Since I posted the question I've been doing some troubleshooting, and I think that the problem is related to the (particular) shield itself.
I bought a second Ethernet shield and another power supply. Both were programmed identically (other than IP/MAC differences) using the library examples. I plugged both into a router, opened two terminal windows and two browser windows, and watched and waited. One of the two (the same one) did in fact crash again. I sequentially swapped power supplies, network cables, cleaned header pins, reseated cables, let the boards run and heat up, threw them in the fridge to chill them, and even swapped the shields with each other. Six hours of this so far but one particular shield had in fact malfunctioned a half dozen times whereas the other has not (despite any changes), so all things IMHO now point to a faulty board.
I thank you Gentleman for your help, and believe that my problem has been solved 