ok spanners, leave the old cards in and see whether it is random or it disappears.
direct x whatever may cause a huge problem, there are thousands out there who will tell you (including me....) but the 7 version solved some problems on my end.
if you find out that it is your graphics adapter, talk to the helpline. had to get rid of a very expensive scsi adapter i bought three months before changing to a p3 system, they both didn't like each other. advise was to downgrade the bios or change it all....as said, seldom the hardware, most of the time the software is the problem.
and this, you can check out when starting w98 via the menu on a step by step way, you can identify where it stops and you see at least where interference takes place (also this may not necessarily be the original problem, but sometimes it is.)
activate the start-up log and see what is written there, if the system hangs, you can see what was noted last.
good luck