My '98 Malibu LS has 109K miles on it. A few weeks ago, the engine was tapping the car was hesitating (while in gear), and that same day, it stalled going down a hill. The oil was 1/2 quart low, so adding oil made the tapping stop. Since then, it has been stalling more frequently. When it begins to hesitate, I have to give it gas to keep it from stalling. It's pretty much a guarantee that I won't make it up a hill without stalling, but it'll also stall on level. I've replaced the fuel & air filters. Any suggestions on what it could be? There are no warning lights on.