My wife actually imploaded two gas tanks on her Chevy Corsica before the service department at the dealer finally replaced the gas cap that was not venting. She had the same symptoms you discribe where it felt like she was running out of gas, so she would stop and put in more gas and was only able to put in a very little, since the tank was full. Then the car would drive good for awhile and then it would start all over again. Next time you lose power try removing the gas cap and see if you now get your power back.
