Engine has 450 hours. New plugs, air filter, gas filter, oil filter. Blew out the cooling fins (oil cooler too). I can cut moderately heavy grass for about 20 minutes and then the engine bogs down. It will not reach full RPM after this incident. After after the engine has fully cooled (usually the next day) it will fine fine at first, and then die again. It's does not skip, blow smoke, or make any unusual noises. It just does not seem to be able to reach full RPM/power until after it sits. This has been happening for some time, but now appears to be getting worse. Any advice would be greatly appreciated. Thanks.