The previous responder probably had it right but I also own an E and
had the identical problem. I think the manual may have a
troubleshooting procedure. I found it quite hard to troubleshoot as
it was very intermittent and seemed to get worse once it warmed up. I
did not work on the motor but did consider replacing it at one point.

I replaced the motor driver IC and the problem went away. Have not had
a problem for several years now.

regards,
Denny