2000 Chevy S-10 Starting Problems

We have a Chevy S10 with low miles. Had no problems until a few months ago. Got in the truck and it wouldn't start. Called a tow company and had it taken to a local shop. They changed the fuel pump and filter. A few days later the same thing happened. Called the shop and they came and got the truck and changed the fuel pump again. Once again, the same problem. They then installed a relay. Did not resolve the issue. The problem is intermittent. The truck will run fine for a while and then, it will not start. It cranks, but will not turn over. If we wait for approximately 20 minutes and try it again, it starts. There is no pattern to when it fails to start. It may happen after a couple of short trips, after a long trip, or after it has been sitting for a few days. So far it has always started if we wait for about 20 minutes after the ititial problem occurs. We have had it now to 3 other mechanics and it does not act up while they have it (anywhere from a 3 days to 10 days). They have checked a number of things but cannot pinpoint the problem. It is very frustrating because we don't know when we will be left sitting in a parking lot somewhere.