1999 Volvo penta 4.3 gl starter issue

Hello all. Well I have ran into something that has me stumped. Last weekend went to crank the engine and could just hear the starter spinning and not engaging the flywheel. I had recently purchased a new starter so new one out of the box, put it in and still having the same issue. The starter motor is just spinning. I had some lag in the starter a few weeks ago but switching to a nee battery fixed that issue and I have not had any trouble until Saturday. Here is what I have done this far. BOTH starters have tested good. Cleaned all connections at the battery and on the starter. (except for the negative battery cable that is grounded in the back of the engine. It is a real pain to get to but am going to tackle that tomorrow. I inspected the ring gear while the starter was off and all of the teeth look fine to me. Again I have had no other starting issues expect for this one. I did get it to crank a few times but then it went right back to spinning. Obviously I need to clean the negative ground post on the back of the engine but what else is there for me to try? This is screaming bad connection to me but looking for any other troubleshooting things to try. Thanks in advance for any and all help!