It's running out of fuel. Check for fuel flow from the tank and through any fuel filters it might have. Sounds like it's getting enough flow to refill the bowl, slowly, but not to run very long.

sounds like you might be right. Dont think I have a filter, dont see one in the manual. Are there any product I could add the the fuel, rather than taking it apart?

What engine? How many horsepower? What make of engine? Need to know, to diagnose it!

ok, after reading some other posts, I added some liquid fuel injector/carb cleaner to the fuel tank and ran it a few times. It started staying running longer and longer, and by the time I finished my first pass on the yard, it was running really well.

Thanks

Gasoline is not a pure chemical. It is refined from crude oil, but there is still some percentage of gum that accumulates in the carburetor, over time. I'm glad that your mower is running well. Some people add carb cleaner to the gas tank used for refilling the lawnmower, and operate the mower on that mixture all the time.