1990 Cadillac Deville

This car had one previous owner, an older lady up until she died, I think car was stored for a while before I got it. It had book and alot of maintanece reciepts and such which shows they kept it up. Seem to have been a very nice car in its day. BUTTTT! The service engine soon light has been on since day one. I get maybe 10 miles to the gallon if I'm lucky. When faced with pulling any kind of incline car begins to gradually slow way down(even dying on me a few times, only for me to restart and continue my journey). Cars behind me get backed up until I'm leveled out some(or better, going downhill. I try to get a running start before I know I'm due an uphill climb, but this is absurd!) While slowly getting up hill sometimes the car sounds like it would like to backfire and feels like it may be flooding because when I push the pedal NOTHING or better yet deceleration.) AND, (very important)during the up hill battles after accelerating it sounds like the engine falls out as it seems to catch a gear that will somehow very slowly push over hill
! I hate this car because it needs something I don't know how to give it! And it kills me with all the fuel I'm burning. I never go anywhere unless absoltely necessary! HELP!