P0171 error code

About a month ago the “check engine soon” light came on. I pulled the code and it comes up with a P0171. I read just about everything there is to read on the internet about the P0171 code and I am so confused now. So far I have changed the air filter, cleaned the MAF sensor, replaced the PCV valve, it was kind of gummed up, put a can of Sea Foam in the gas tank and checked all the connections and hoses. I can clear the error code drive the car all over town for days and the check engine soon light will not come on, when I get on the freeway it will come after about two or three miles of driving on the freeway. The car runs prefect and is in excellent condition. I am at a loss, I do not know what to do or direction to go in now, I am an old Vietnam veteran on a very fixed income.

Thanks for any help or direction you can give.

Randy
Tacoma, WA