Troubleshooting a vehicle online can be tough because you have to assume so many things. Is the troubleshooting on the other end being done correctly? Is the advice you're receiving helpful? You have to assume other systems are functioning correctly. And are quality parts are being used. With that said, I would make sure the following tests are in the normal ranges.
1. Engine compression
2. Cylinder leak down
3. Smoke test for vacuum leaks
4. Cooling system is functioning properly
An engine needs four things to run: Find out which one you're missing and concentrate your efforts there.
1. Air: No major restrictions on the inlet side of things. All couplings are tight and on correctly?
2. Fuel: Fuel pump, fuel pressure, and injectors ok? Gas is fresh and the fuel tank is in good working order?
3. Spark: Are all ignition components operating as they should? Do you know how to test? Are you using quality components instead of the lowest buck option (which seems to be a favorite among some owners)
4. Compression: If the compression is in the normal range you're good.
The 5th thing your engine needs to run is Engine Management. Were any modifications done to increase air, fuel, or spark at idle, and cruise? Do you have the ability to view logs, fuel trims, load, O2 reaction would be helpful but now I'm probably getting ahead of myself here. And it's ok to bring the car to a shop, or dealership if you feel you don't have the experience or tools to troubleshoot it yourself. Better to fix the car than have it sit because it sucks to drive.
Good luck
Michael Plummer