This wouldn't happen to be a new Prestolite starter motor would it ? I have just had to replace 2 brand new ones on my 3.5V8i. First one packed up after about 6 weeks, second one only lasted 3 weeks. I have just had a new Bosch one fitted and, hopefully, it will have sorted the problem. My problem started with the starter motor spinning but not engaging the flywheel. Then it would not even spin at all. When I took it back the guy in the parts department told me he had been having trouble with new Prestolite Starter motors.
HTH
Ivan