Engine repair

Starts put wont stay running. Car will start and run great and then all of a sudden it will die with no warning and will not restart. I took it to a diagnostic car center and they first told me it was my harmonic balancer bad and it was but the problem was still there after putting on a new one. Diagnosed again and was told it was the positioning sensor and so I replaced and still no fix-Then I replaced the computer and it ran for two weeks and now it has started again. I have now replaced the following things-timing chain, magnet and camshaft sensor, crankshaft sensor, fuel filter, ignition module, coil packs and battery ends. What else can it be? I am so lost now. I need someone with answers.