I have a problem with my Gravely proturn 260, the engines seems to want to bog down after running for more than a half hour.

I can be on black top and it still bogs down, I have changed air filter, fuel filter, spark plugs, and I put HEET in the gas tank to make sure there is no moisture.

I hear their may be some sort of EFI chip problem? Anyone else having this issue?
Brian