So ...

I uninstalled VFP then reinstalled it into a new folder
and then reapplied Service Pack 2. Rebuilding the project
and got about three hours of good work done, then the blue
screen again. With a sequence of error messages I had not
seen before. I took some notes:

[1] Blue screen of death, and I didn't catch the text on
the first screen. I pressed the 'any' key (the big
long one at the bottom) and got
[2] VFP6 - Caused an Invalid Page Fault in module USER.EXE
at 0003:0000a5d4. Clicked 'Close' and got
[3] Kernel 32 message in a window sized like the GPF box
we all know from Win3.x. I think I clicked Ok and got
[4] Blue screen: WINDOWS - Fatal exception OE has occurred
at 015F:BFF9DBA7. Pressed the any key
[5] Kernel32 - GPF in KRNL386.EXE at 0001:000075a2. Clicked
'Close'
[6] W98 box: Explorer has caused an Illegal Operation and
will be shut down. This box disappeared when I moved
the mouse and was replaced by
[7] Same as [4]. Same address and everything. Clicked close
[8] Same message as [2] except it referenced Explorer instead
of VPF6. With the exact same address. Clicked close, and
[9] Saw the empty shells of the VFP windows I had been using.
No titles or menus on the main window, though the bars
were there, and an empty project window inside. Plus I
think the command window's shell was there. Ctl-Alt-Del
did work, and showed a task list which did not include
VFP6. Pressing Cancel got the blue screen back and the
3 finger salute rebooted me.

So after all that, does this sound like a video problem?
I have an SIS 5597 video card with 2Mb ram, with a Daewoo
monitor. W98 says no driver files are required (or have been
loaded) and it has worked ok so far. 128Mb of Ram and about
5Gb of free disk.

Again, thanks for all the advice. I hope this leaves a
record for someone in the future who may gain from it all.

Bob