Error Message: DIAG 66: Drive Malfunction

I have a 1990's model Beckman Optima XL-100K centrifuge. Had been running well for over a year. Recently, the recent runs have shut down after about 5 minutes. The unit will restart and shut down about 5 min after hitting speed. There are no weird noises that I can ascertain. The centrifuge has been shutting down with the above error message. Among things I suspect is that the oil in the drive needs changing. I looked on line and there is no informatiion that I could find. When I querried Beckman about buying a service manual, the price was $3,213. Seems a bit steep for a nearly 30 yr old centrifuge. Any ideas on how to proceed? What can a reasonably competent user investigate and service to correct this situation.