W98 won't recognise and set-up hardware

Any 'old school' experts out there please...???

On an old but generally reliable Windows 98 machine:

ISA Sound card stopped functioning (no sound) - device manager suggested a problem - reinstall driver.

System then asked me about reconfiguring LPTs which I didn't want to do, so cancelled.

On re-boot the system then lost all access to expansion cards (display, SCSI, ethernet) - display was left functioning at VGA.

Managed to get display to function by swapping the AGP display adaptor (card) with another similar unit and re-installing drivers.

Unable to get W98 to recognise PCI ethernet and SCSI adaptors - also the USB connection gone (that's on the motherboard). LPT1 and printer still works. (LPT2 [ ISA card] not used, so not tried, but appears ok in device mgr. - I've tried taking it out altogether, but that doesn't help.)

Tried different ethernet cards - no luck.

On manual harware install attempts, device manager will either show no expansion hardware, or device with problem. (Most motherboard items, except USB, seem ok - e.g. Drives A,B, C,D, F, PS2 mouse, k/b.)

In 'safe mode' most things show as functioning in device manager and pretty much all looks present (whereas it isn't in normal mode).

SCSI devices show on boot-up and can access the SCSI at DOS level using W98 start-up disk (but that doesn't help much).

Have tried all the usual add hardware functions and have tweeked the registry setup flags (as per MS support instructions) to cause W98 to re-run hardware detection. It starts, gets to about 3-4% on the progress ribbon, that panel disappears and then the system just loads W98 still without hardware. (Tried lots of times).

I have a back-up, but it's on an Iomega Jaz (via SCSI), and I can't access that in W98 now. 

I'm loath to run W98 set-up if I can avoid it. 

Has anyone any suggestions as to how I might get W98 to recognise and set-up hardware please?