Thanks Joska, was about to try your suggestion then I thought about trying a floppy that has an executable bootsector, DBUG's Gauntlet 2 (patched) has a anti-virus bootsector (fcopy one i think), anyways the bootsector did indeed run, displayed its message then DBUG intro came up but then onto GEM Desktop (should've loaded gauntlet).
Switched it all off left it a few moments, now it all works again with no 4 bombs!
Surely a executable bootsector couldn't 'fix' this type of issue as it's been happening for days.
Unless of course it's one of those random 'did nothing' just works for now type thing......
p.s. RAM tester (YART) worked fine, so did GEMBENCH 6 (all when I was getting the 4 bombs with disk in drive). I wonder if the PSU is dying this would be the sort of behaviour expected?