Cranks but will not start

The short story is, a month or so ago, my car stopped starting consistently. It would crank, but not start. No clicking, just the sound of the engine trying to turn completely over and start, but never getting there. So I took it to my service place (not a dealership) and he said in these situations, they usually start with the starter. They replaced that. $470.00. Two days later the same issue. Again, it was not all the time, probably 35-45% of the time. Enough to be really annoying. Sometimes I would sit for five to ten minutes trying it sporadically, eventually it would start up. I took it back. Then they said it could be a relay. They replaced that. $90.00. A few days later, it started doing it again. Same consistency. I took it back. This is three days ago. Now it is a purge valve and the ignition switch. $260.00. Replaced those. Tonight it did it again. I sat in a gas station parking lot for ten minutes and tried it a few times and finally it started up. What gives? When I picked the car up, the guy said the only thing left that it could be is the computer. I do not know what to do. I cannot have unreliable transportation. Please help if you can. I am already out $800.00 on this. Thank you in advance.