1992 Volvo 740 Starts erratically

I bought this car about 5 mnths ago and it has been giving me problems from the start. The frst time I drove it I got stranded at the grocery store. We replaced the battery and it seemed fine. I had the alternator checked and wad told the voltage it was putting out was low, but acceptable.
About a month ago I started having more problems. I went to the post office and came to no start, got a jump and went up and had the alternator checked immediately. Tests showed it was dead,
My mechanic dicovered that the previous owner had replaced the original alternator with one designed for the turbo 740 and rewired it so my mechanic replaced it with a turbo alternator.
The car started up fine and I took it up to make sure
both battery and alternator were OK. Both checked out fine. I drove 7 miles down the road and the car cut off while I was driving. The car would not start, but took an immediate jump.
The next morning the car would not start. Recharged the battery and car started. I drove 2 miles to Walmart and back. The next morning the car would not start.
My mechanic pulled the new alternator and replaced it. Recharged the battery. He drove it all over town. Turned it off and on without problem. He brought it to my house and, as we were talkng, the car cut out and would not start again.
He checked both the alternator and the battery and they are working fine.
What could be causing this problem?