1990 Cutlass Supreme

I have recently bought a 1990 cutlass supreme with about 106000 miles on it, and have run into several problems. first the car was dieing at stops and rpms jumping a few hundred, i checked the transmission fluid and it was low but not empty. i put a quart in and the problem was fixed. but, i was driving a 2 hour drive to take my brother to work and on the way back my car started vibrating all of the sudden... it sounds like somethings hitting the inside of the fender well... when i pulled over oil was leaking from somewhere and i found out the oil pump drive o ring was bad, cost me 266 dollars to replace that and a leaking temperature sensor....fixed the oil leak. but the noise continues and rpms are about 500 on average, i have to keep the ac off at stops or it dies, and shifting sounds like its straining.. and the gas mileage is all of the sudden horrible like about 90 miles to the tank.... anyone have any ideas?