[mikester]

Have you tried draining the gas you have in the tank? If you're using gas with ethanol in it and it sits for a bit, especially in unsealed lawn mower gas tanks it will start to absorb water.

Then; as you start your mowing session you might add some new gas or it quits and you figure it needs gas so you add some. That makes things better for a bit but that old water holding gas is still in there. The mower might run but it'll run like crap. At some point the old gas will absorb so much water it won't run at all and it'll appear milky white. Your symptoms sound like you should drain the tank and check.

If it comes out with a bit of milky white liquid from the bottom of the tank (which is where the pickup is) then I bet with a 100% fresh tank of gas you'll be good to do with a mower as good as new.