Could be your flywheel is damaged..

Had the same problem on a Volvo 940, most times it started ok, but once in a while it you make the grinding/set your teeth-on-edge /scare the dog sound.

I had to remove the gearbox to change the flywheel oil seal so I took the oppurtunity to look at the starter..it was ok.. I then took a look at the flywheel and there was visible wear along one portion of the gears. Changed it out and the problem was solved. I think the trouble starts with a flaky starter which then grinds the flywheel gears down. The starter gets repaired but the damage is done.

Later on I was told that since 4-cyl engines usualy stop in one of two position.. 2pistons up and 2pistons down, it would be possible two re-install the flywheel 90degress off (as compared to the current position) this would take the damaged gears out of the area where the starter would 'catch' . This wouldn't have been possible on my car as it ha a crank position sensor on the bellhousing,,I would have hadd to drill a new hole and rebalance the whole thing,.

Best of luck
Anil