A lot of B&O products will go into "Standby" if there is any kind of power supply issue with a view of protecting the unit itself.
The fact that both units went to "Standby" means an actual "Standby" signal was sent as if you had switched it off from the remote.
I am assuming that the Beomaster works fine if the Beocord is not connected to it. If so, then the beocord is likely to be drawing to much current. So anything which is likely to put extra torgue on the motor will draw more current. You said that tapes sound a little slow, so this is also likely to mean it drawing more current.
I would check the motor and associated drive again looking for any particular tightness.