I think 4.9v is OK.In my case,it was about 4.8v.But I still think you want to be sure capacitors in PSU are healthy.I heard SCSI bus took almost 1Amp when many lines change between 0 and 1.makes ripple on power line if capacitors are weak.When I got problem like yours,floppy drive was removed and only CF-SCSI drive was attached,so load on PSU had to be lighter then usual.