As billchase said, it's more likely to be an intermittent power problem.
If you have the option, use a different floppy drive power connector, most power supplies have two. Sometimes the crimping of the socket pin onto the wire can go dodgy.
Resolder the joints on the 4 pins of the floppy power connector on the PCB, as these can develop solder fractures due to mechanical stress from the power socket. You may have to partially dismantle the drive to access the bottom of the PCB.
I'd also replace the 34 way ribbon cable, as those IDC crimps can develop intermittent problems when you least expect it.
