Ferris Procut S not running full power

[Outdoorstom]

I'm hoping someone here can help me out. The 27 hp Briggs and Stratton on my mower recently lost some rpm's while mowing a steep bank. It still cuts, but not at full power. I've replaced fuel lines from the tanks, removed the tank and flushed it, replaced the fuel pump, and replaced the fuel filter. The carburetor has also been cleaned. I mowed today and it was at full power for 5 minutes, then back down to the lower rpm's. Does anybody have any idea what the problem could be? I'm at a loss! Thanks in advance!

 

[BlazNT]

Get 2 inline spark detectors and fit them to the mower. When it starts slowing down check for spark for both spark plugs. Let us know what you find.