Well, I've tried all sorts of stuff and while for a short period it seemed things were more stable - replacing the PRAM battery has made the "stutter" startup go away, I still get random sudden crashes. The PSU looks ok visually, it seems not to be overheating or drawing excessive current. I've cleaned it, but it wasn't significantly dusty, neither were any other components. As the first symptom is actually sudden loss of video signal, and running iStat menus appears to show strange spikes in GPU usage instants before, I'm tending to look at the video card. It is an Apple-supplied ATi HD7550 upgrade card, which is actually not "supposed" to be supported on this early 2008 Mac Pro (but it has been working fine for several years, and a friend of mine has one in a 2007 Mac Pro). It is a very tight squeeze and possible it is not thermally fully compatible. However there is no correlation between heat and crash - it can easily happen minutes after boot - and anyway I'm not seeing anything approaching high temperatures. Lightroom really doesn't tax this system much.
However, it is getting to me concerned, these forced reboots are not necessarily good for other components such as disk drives. So, I'm now looking at options.
Within my budget, there seem to be too poor choices - either a fully maxed out Mac Mini (with 1Tb SSD, i7, 16Gb) or the lowest config Mac Pro. The Mac Pro is currently being sold here in Switzerland at some retailers at CHF 3000 (so pretty much $ 3000). Apple wants CHF 3400 for it. The Mac Mini, from Apple, is CHF 2199.
My main use is Lightroom and similar applications (Iridient mainly), and diminishing amounts of Photoshop. Also some InDesign. And of course general home/office stuff. I don't do video and never will. My largest image files are MF film scans which can easily reach 1GB, but mainly I process 16 Mpix ORFs and Sigma Merrill/Quattro. I currently have an internal 4TB SoftRaid volume (2 WD Black 4TB) backed up to a Drobo 5Dt (on a USB3 PCI card). Oh, and a Eizo 270Cs and Quatto 240LE, which are the two good reasons I am not considering an iMac. Not a fan of glossy screens.
I'm tending to think that the Mac Mini is the better bet. Apart from anything else I'd be immune from temptation to upgrade it. But 16Gb may be a little low. But from what I can see, Lightroom makes very little use of the GPU, so I doubt that the graphics cards in the Mac Pro would be of any benefit.
So yeah, two bad choices - thanks SO much, Apple. Any opinions on which is the least bad ?