1998 Chevy Truck 2500 won't start

I have a 3/4 ton 4x4 chevy truck that has 200,000 miles. I recently put an engine out of a wrecked 1999 Suburban that had 69,000 miles. It has run great for a year but in the past few weeks it would not start. The fuel pump was whining so I replaced it and it ran fine for a month or so but now it won't start again. I can get it to start with ether and it will run all day long but when you turn it off it won't start. I had it in a shop and all test ok, spark, fuel, and sensors. He thought it could be the computer but did not feel strongly enough that this was the problem to incurre the expense. Any ideas?