2002 Pontiac Grand Am Problems

For the last 6 mths I have had problems with my Grand Am (3.4)liter 6 Cylinder. 2002 It started with all my gauges acting up...my low oil stays on all the time. my seatbelt light stays on even when fastened. my low coolant light comes on periodically. I have taken it to my mechanic 3 times in the month of July. Had problems not starting leaving me stranded. First time i was told it was the wires running from the starter all the way up had burned. We replaced these only to be back in the shop 2 weeks later for the same problem. No start! Again no apparent problem. we later discovered it was the battery....It was bad replaced it seems fine now but my lights on my dash keep comming on and they drive me crazy because i don't know why. Now i am seeing a decrease in my fuel mileage. I could get 250 miles to a tank of gas around town. It decreased to 200 and now its 150. What could be causing my fuel to decrease so much? What could be causing my guages to be acting up too? Please HELP! Repair bills are getting outrageous!