[...], but the second time, I happened to have a dos floppy
   in the a: drive while trying to boot NetBSD-1.0 off the hard drive.

   [...]

   Just then I noticed that it kept on rebooting as if someone kept hitting
   the reset button or <crtl-alt-del>.

I remember seeing exactly that symptom once, and fixing something at
the time, but I sure don't remember what it was!  What version number
does the boot block on the hard disk print out?