I presume by now you will have solved the puzzle but if not..
Do the same test whilst the motor is wired into the car loom but not connecting to the wiper arm mech, remembering to earth it to the car body or earth feed.
Any luck ?
If it the motor turns ok then maybe a little lub on the mech would help.
If not check the earth is good ( i,e, use a star washer under the earth point to make sure it will make good contact with the car )