Then, several restarts later (due to my lack of speed on the DEL key), I managed to get at the BIOS. I decided to start from scratch, and loaded the safe defaults. Then I went through and tweaked it carefully (and very conservatively). After that, it booted fine. To be sure of this, I rebooted it (both warm and cold) about 30 times. All good. Then I left it to its own devices with a bunch of stuff running for a couple of days. Still good.
Then I tried again to install my Solaris-11 system, hoping that it would work on the new setup. No joy at all. The installer crashed, several times, in exactly the same place as before (i.e., long before getting close to actually installing anything). I really wanted to be able to use ZFS, which I think is a wonderful file system, but it seems impossible for me to install Solaris on any reasonably up to date hardware that I have available. I think I'm over this now and will wait for some other operating system that I can bear to use offering native ZFS.