Does anyone have any idea what is wrong with my car?

It is a 1999 Mercury Sable. When driving uphill it will sometimes feel jerky, as if it is about to quit; this problem is more noticeable when driving slowly uphill. It will also occasionally stall at stoplights, and the clock will be reset when I turn it back on. My mechanic drove it and couldn't get the same problem to occur, so he couldn't figure out what was wrong with it.