All I can offer is personal experience on this one...on a 1984 GMC Sierra
C1500 6.2L pickup truck...
1. Is your starter original?
2. Have you taken it off and looked at it?
I don't know how much of a difference the third support bracket makes. I
tested my starter with it off and didn't see a problem. The first two bolts
are very important. Make sure they haven't sheared.
The starter on my truck had ground badly for years. It wasn't until the
solenoid burned up that I decided to find out why. I dropped the starter to
find that the casting around the drive assembly was broken and resting up in
the area where the starter's shaft normally is located. It had just cracked
off.
Upon dropping the starter I had it rebuilt by a good independent automotive
electric shop. What I got back was a night and day difference from what I
had brought in for repair. (Yes, it was the same starter...just a before and
after.) Putting the "freshened" starter in place resulted in a noticeable
difference. Starts were stronger with much less grinding. (I had to have the
flexplate replaced in the truck as well...now it starts and cranks very
nicely.)
I don't recall ever having any shims or other adjusting devices in place on
my starter. It was just the three bolts and any applicable wiring.
William The Guesser