Thanks everyone for the input so far.....
I had a 560 gas on the tiller running it with no problem so the 656 should handle it too. I sold the 560 to buy a loader tractor a few years ago. I also used my U504 on the same tiller for a whole year before I got the 656. Granted I couldn't get it as deep, but it did the job for the duration.
I can kick myself for not thinking about the coil issue that it could be. I had a coil go bad on the 504 years ago and I had a real struggle finding that issue. It acted the same way.
What I found on that one was that when it would heat up a tiny hairline crack would open up (expand) on the top of the coil and arc / short out causing the shut down. I never would have found it if I wasn't playing around one night plowing after dark and turned the headlights off (I always liked to see the flame coming off the stack - LOL). I noticed the arcing of the coil in the dark.
Side humor: My brother and I used to play chicken with each other on our 3/4 mile long rounds at dark plowing in the moonlight with the lights off of our 706 and 806. Crazy but it is fun listening for the other tractor (you notice the slight rpm differences first) then notice the stack with a flame about a foot off the muffler coming back in the opposite direction. Being that I am the younger brother, I was always stuck with the 706 (gas) and he had the 806 (diesel). It did turn at slightly higher rpm's so I had fun lapping him after quite a few rounds. I would always wait until I was practically climbing on the tail wheel of his plow before I would switch the lights back on and scare him.... Younger brothers revenge!!! You wouldn't dare do something like that today! Surprised we are both still alive....
Anyway, I replaced the coil on the 504 and have never had an issue with it since.
I will switch out the coil and see if it helps. If it doesn't I will replace the plugs with the proper plugs per spec if they are hot ones.
Thanks again everyone,