BR, for a saw of this age that is very good. However I am a little puzzled.Hot the compression usually causes the pressure to drop.However I will say that there is some unburned oil in the cylinder.How about the gas tank vent? Going back to when the saw quits, does the engine race high for a second then die? How about checking for a bad crankshaft seal.The seal gets hot expands and the engine looses vacuum and dies out.Next time it stops, pull the plug and set it on the saw and pull the rope. If you have spark then it is more than likely not the coil. I am running out of theories.Next time it quits pull the air filter of and put in the carb some mix. See if it starts, then dies. Here is a long shot,underneath the carb there is a vacuum line going from the bottom of the carb heading to a small port in the crankcase. Check for a cracked line.My 041 has this but I am not sure about the 51.Whew! Ken