my hot caddie won't start

my cadillac 89 el dorado idles really high for about 15 mins on a cold start, and will not start hot, just clicks. i am also having a problem with it taking off when i stomp on the gas (hesitates). my service light comes on intermitently and occasionaly the engine will sputter if left to idle for awhile. my gas milage is also way down from where it should be. the shop says i need a new start but i believe there is more to my problem, please advise. pants1