RD53 Restoration

I have posted before about an RD53 I am trying to get back to working order. After unsticking the heads I thought I had a working disk as I have been able to format it and do an image restore of the VMS installation media to it. However, intermittently it has been failing. When this happens the heads completely fail to move when I power on the disk, so the controller and firmware cannot even determine its size. After a few moments the disk spins down. As I said, this is intermittent, sometimes it will work OK. A friend has been helping me and he gave me a complete set of the three boards in the RD53 from a known working disk (he can't give me the disk for reasons not worth going into). I changed all three boards and the drive still completely fails to move the heads, in fact now the fault seems permanent. My friend suggested that the positioning coil may be faulty. He measured the resistance of the coil on one of his working disks (at the plug that goes to the coil from the motor control board), it came to 3 ohms, mine also measured 3 ohms. I measured the voltage at the plug going to the coil, one reads 2V the other 3V, but when it works they both go to about 5V. Can anyone suggest what might be the problem? Thanks Rob