Good news to report on this. Cleaning the commutator did the trick. No more popping and tripping even at full load. I will still need new brushes shortly but at least I now have the time to research and maybe even replace the motor with something better.

After doing the first clean up with emery paper, I read that using emery is not such a great idea since the particles can get between the segments and cause shorts. However I thoroughly cleaned between the segments with a soft brush on a dremel and in fact the gap between the segments is big enough easily see any major debris. Just mentioning this in case someone is reading this thread with the same problem.

I actually now think the major issue was carbon build up between the segments.