I bought a house on 1.3 Acres with a sloping back yard. Lots of trees and landscaping means there isn't a lot of striaght cuttings. I got a commercial walk behind. Love it. Takes 1h15m to cut and looks better than every other neighbor's lawn who use lawn & garden tractors.
Anyway, it sounds like the motor is overheating if it cuts fine for a while then loses power. Change the oil, plug, wire. If that doesn't help, you can look into motor work, but honestly, you can replace it with any other used $500 mower that will work better, or a $1000 Sears special. Lawnmoers of all types have very poor resale value which makes working on them pointless.
