my 1995 olds eighty-eight royale started sputtering after i put gas in it but not immediately after getting gas, i drove it for about 20 minutes, parked it for a couple of hours and then it sputtered and cut off and will not start again, i checked the fuel pump and filter, i did the test to see if air was in the tank by putting some in a jar and watching for air bubble to form, it didn't. any ideas? oh yea, it back fires when i try to start it also.