When the starter goes, do you hear a soft electric motor whine or a loud screeching like someone trying to restart their car. If it's the former, your rebuilder has a bad batch of Bendix's, if it's the later, you have a bad spot on your flywheel. I've had 13:1 race motors that would eat starters like snacks, had to go to a custom built with a special Bendix to get them to last. I'm currently running a 524 with 10:1 compression and my timing is locked at 34 degrees and the worst thing that happens is my starter will crank slow. I always turn the key first before I shoot it some gas to get the motor moving first.