Some thing worth considering if its a star 3 phase winding, is the joints where the 3 windings join to gether good? I had a similar problem with a 3phase brushless motor & was intermitent poor connection.

 

Some thing worth considering if its a star 3 phase winding, is the joints where the 3 windings join to gether good? I had a similar problem with a 3phase brushless motor & was intermitent poor connection.

This was the 3 phase brush less motor, have marked where the center of the star windings are terminated & were intermitently faulty.

 

This was the 3 phase brush less motor, have marked where the center of the star windings are terminated & were intermitently faulty.View attachment 368056

I tried swapping the motors, it's the same issue as before.