I believe this is commonly because potentiometers on the motor control have become unbalanced and need adjusting. For example as here
https://www.b82maidbronn.de/lx200.htm
There was someone in the UK (Alan Sickling) who used to know all about these. His email was alan (at) sickling(dot)co(dot)uk but this was from over 10 years ago now
Cheers
Robin