start up

ok here goes, 96 gt 2v v8 when i start the car it cranks and runs at about 4 to 500 rpm for 5 to 10 seconds then it pics up to about 1500 rpm and this is only after the car has sat overnight. my question is is this normal car has 113670 miles on it. i recently changed my fuel pmp but it had started doing this before i changed it. also can you change the oil pan gasket without taking the engine out i haven't got under the car to look but was told by dealer that you had to.