Small Engines (Lawn Mowers, etc.): Murray Classic Mower, octane booster, push mower

Question
I have a murray 20" classic push mower, 6 years old.  Well taken care of.  It runs good for about ten-15 minutes and then starts running rough, almost about to die.  If I pull the mower back towards me quickly it will die.  I have replaced the plug, new air filter, took the carb apart and cleaned everything with carb cleaner and compressed air, added a little octane booster with fresh gas, changed the oil.  About a month ago it seemed to run at a higher than normal run speed when we first started it for some reason, maybe for 30 seconds or so and then it would settle back down while I was mowing.  Could my governor be hanging up?  Bad gasket?  Help.
Thanks.

Answer
Hi Gregg
Check the magneto arms for rust and/or trash buildup and also the flywheel magnet. It sounds like the magneto is breaking down after it heats up. Dirty magneto arms and flywheel magnet can cause this. Make sure the linkages and spring are installed correctly and that they move freely. If they do, then we can assume that the octane booster is the cause of the higher rpm. Hope this helps.