I can replicate this in my driveway. I ran it 30 minutes at 1800 rpm (neutral) and she started choking and sputtering. I hooked up a timing light and the mark was bouncing all over the place. When I first started it I took the timing and she was sitting at 8 degrees BTC 900 rpm and approx 2-3 degrees before the scale (approx 16-17 BTC) at 1800 rpm, 20 minutes after running I took the timing again and now she was just touching the timing scale 14 degrees BTDC. 10 minutes after that she was choking and sputtering and now the timing was bouncing all over the scale including ATDC. Why would the timing be changing going the same speed? I also tried icing down the ignition module and that didn't help. New coil. Timing chain is 3 years old. Ignition Module is also 3 years old. 
