DGT-6000 (917.276380) engine cutting out

I have a DGT-6000 (917.276380) that wouldn't start this spring. I was pretty sure that it was the fuel, so added fresh fuel, new fuel filter, and new spark plugs, to no avail. So, I hired a guy who fixes lawnmowers on the side, and he drained the lines and tank and got it to start. He also serviced it too, and said something about the float when he returned it, but I'll be darned if I can remember what he said!

OK, so I'm mowing my WAY too long grass, and have the mower deck as high as possible and it's mowing fine. But, then I get to the drainage ditch by the road and when I traverse it with the right side being the low side, the engine would just cut out and die. Mowing it with the left side being the low side, it mowed fine!

Then, I started to notice that if I got into bumpy ground, it would start to cut out and show black smoke out of the exhaust.

I checked the seat cutout switch, and even made it to where it was pushed closed (like I'm in the seat) all the time, and I still get the same problem. I checked all other electrical connections, and found nothing loose.

I believe the symptoms could be either the carburater or something loose in the electrical, right? A new carb costs over $200, so I don't want to "guess" that it's the carb and be wrong.

Any suggestions, ideas, or something I'm missing? It's VERY frustrating, especially after I just paid to get it running from the winter and serviced. Grrrrrrr

Thanks to all in advance!
Ron