Elderly mother has a 98 Buick. Garage kept. The other day battery was dead and needed jumped, so I jumped it & got it started. The engine loped, would race then nearly die, but eventually smoothed out at idle. She went on to Wal-Mart, Dad stayed in car so they wouldn't have to shut it off. Mom says today that the car loped & wanted to die all the way there and home. Dad says idled ok in the parking space. Bad battery? fuel pump? Suggestions?