No Power

'89 Cutlass Ciera Cruiser Wagon, 2.8L, V6, ~72,000 miles has a problem with lack of power. Car had sat for over a year with less than a tank of gas and undriven. After filling the gas tank for the first time, approximately after 40 miles, the engine lost almost all power. It would run better at an idle, and when pressing further on the gas, the more it would lose power. Of course, it was much worse to get up a steep incline. The fuel filter has been replaced, as well as the spark plugs at this point, but this doesn't seem to fix the problem. The car still hesitates, misses, and loses power when idleing, but especially when accelerating. Additionally, the engine runs worse, and has a greater tendency to die after the engine has become hot. Furthermore, the engine fan has not kicked in even when the thermostat reads 220 degrees. I have only owned the car for a week, but it has yet to overheat even in 80 degree weather. Could these two problems be related in any way?