My starter spins, but doesn't crank the engine

My starter spins, but doesn't crank the engine. Car has always had trouble starting, believed to be a fuel and/or electrical issue that I've been trying to eliminate. Lately the starter has been starting to grind as it starts, we found a ground wire attached to the starter bolt had come uncrimped, therefore we assumed that was my problem.

However, a week later, it's back to free-spinning. It didn't grind like the last one did, just a little as it seemed closer to it's death when it stopped engaging the flex-plate. It started with minor difficulty this morning (kinda the usual) but ran like a champ for a couple hours, starting perfectly fine as its warmed up. Ironically I was picking up new injector o-rings, as I was planning to clean my injectors hoping to solve part of my problem. Just two hours after my commute, I tried starting again as I usually do. Typically it would fire up with little hiccups, but it seemed a little more violent trying to crank over (heavy shaking). Typically giving it a little throttle helps the process, this felt like it wasn't getting fuel. Then, mid-crank, the starter started free-spinning, now it'll catch for a split second before free-spinning, every time, just like the last one. My guess is that the starter is potentially good, but something is depriving the starter solenoid of power. This guess being that my last two replaced starters tested good in the parts store.

A little history about this car. I bought it from a friend who bought it as a fixer upper. Many things have been replaced since we've worked on it. We've replaced:

Tried replacing the relay and checked fuses, nothing. It's recently been re-timed at 20 degrees and I use 89 or better fuel most of the time.