95 Ford Taurus starting problems

My 95 Ford Taurus starts and immediately shuts off and the battery light comes on. I sometimes have to start it several times and rev the engine to keep it going. This usually happens after it has sat for at least 1/2 a day but not always. My mechanic has tried replacing the relay module and suggested I turn the car on and wait a second or two before starting it. This worked for about a week but now the problem's back. Any suggestions? I have about 97,000 miles on it.