[Rhgbristol]

You could try starting from stationary several times and closely observe what happens.

With the ac supply, it will be pot luck on which part of the sine wave the motor starts rotating on.

Usually if running backwards, within a second or two some sort of simple cam intervenes and as you say ‘bumps’ rotation, effectively stopping it and causing it to bounce back and rotate in the opposite - correct - direction.

It should be possible by watching multiple starts from stationary to see what sort of bump mechanism you have, and it may be that just a very simple adjustment or lubrication is all that needed to get it working properly again.

Hope this may help.

I have come across this problem on many older Metamec and Smiths clocks.