this lens is a little bit too sensitive – after a certain time the inner barral will have an increased backlash – which will cause a less accurate focusing.
the solution: You have to change the cam followers.
Here are the prices:
So, 10 USD for each piece. thera are 3+2. Plus post. Spareparts: 50 USD….. not too cheap.
If there would be all 3 resp 2 in the price – then would be OK.
But ithout this, You will have always focusing problems.
I will try to make this parts from PTFE
DIGITAL CANON repair blog 
Hi, i wrote from Argentina, have a Canon 50mm 1.8 Mark I. The manual focus work fine, but when turn AF on, the lens go to front and back continuous and cannot make focus.
In another blog i see that this model have a internal potentiometer to control speed of motor, but cannot fix modify angle of this potentiomenter. You have a similiar problem with another old lens canon ?
Regards!
Hi, I dont have too much experience with that lens – excepting one piece, which is mine.
case the lens cannot find the focus normally means, that the incremental sensor is not giving the signal – in this case i think is a cutted wheel on the motors axis. try tocheck the feedback signal with an oscilloscope.
Thanks for you response!