Kohler Command 16hp. Loses spark after running a couple of minutes. Help!

I tried replacing the coil with a parts tractor (same engine). Still does the same thing. It started last week when i first got it out of winter storage. it ran fine for about 45 minutes, started popping and backfiring then ultimately stalled. Quick spark check showed no spark, but it returned after it sat for about 15 minutes. Then it ran until I finished the rest of the yard, another hour or so. Did it again when i went to mow this week, but this time it won't stay running for more than a couple minutes at a time until it cools down. I immediately thought "bad coil". Swapped it out with another that came from a good running engine and changed the little black box (rectifier?) that has 3 wires running to the coil plus two others (white and white w/ red stripe). Still doing it. What else could it be?

Sounds like from your description that black box is the spark advance module. Lose of 12v power to it will cause lose of fire. Check all electrical connections for corrosion or any broken loose connectors.

Thanks ILENGINE. I kinda gave it a once over, didn't see anything obvious. I'll run down each wire more closely and see what I can find.

I think i saw on a U Tube video someone on there with that problem and he shows u how to fix it
Might want to try UTube and search there ...
I just stumbled on it when i went there searching for help to my no spark problem ....