90 5th Avenue New Yorker

We just purchased a 90 New Yorker with a 3.3 automatic, 90,000kms. It was used by a little old lady (really) and is in mint shape. It stalls when driving along occassionally and will not start for a while. We had it to the shop for 4 days and it started every time for them. The previous owners had another computer installed as the check engine light would not go off even tho there was nothing wrong with it. The owner's son said they never had a problem with it stalling. We have heard the words cam sensor and crank sensor. Could one or both of these be the problem?