I hope I phrased this correctly. lshw gives:
description: DIMM Synchronous 400 MHz (2.5 ns) product: M378B5773CH0-CH9 vendor: Samsung physical id: 0 slot: DIMM0 size: 2GiB width: 64 bits clock: 400MHz (2.5ns)
And indeed the memory speed is set is set to 800MHz in the BIOS, which I think makes sense since it is a double rate.
On the other hand, Googling strongly suggests that to this product number corresponds the PC3-10600 type, which is 1333MHz, not 800MHz. And this seems to be confirmed in the BIOS, where if I select Auto for memory bus speed, 1333MHz is selected "based on SPD settings".
However in the latter case, the computer does not boot, i.e. the kernel panics, complaining that something attempted to kill the Idle process.
So, I am I am beginning to suspect that I have been given defective memory, the technician that installed saw this, and lowered the bus speed. Is this a possibility?
The suggestions below have been very helpful, wish I could accept them both. Well, before running the memtest, I had the idea to reset the bios to its default settings, which I had not done on this computer. What do you know? Both the bus clock was set to Auto, i.e. 1333MHz and the computer booted normally, and seems to be able to pass the memtest (didn't let it finish). Before, I could not even get GRUB to load sometimes, and never the system.
So although I was paranoid about being scammed, it was some setting that the retailer en(dis)abled that caused the crashes. What it was, and why they had changed it will probably remain a mystery, since the days when I could understand HALF the options in my BIOS are long gone. :-)
I will post back if there is any more instability. Many thanks.