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Has the shop put the computer software on the engine to check the parameters and trouble codes? If there are codes they will show on the printout. If there are no codes and all the parameters show normal then the problem would be in the fuel system.
For this you need to be able to monitor and test the fuel pressures.
Has any of this been done?
Lets see where the diagnosis goes.
In my experience fuel restrictions and failing fuel pumps cause the type of problem you are describing if there is no trouble codes.
I wasn't on when your request came in. Did you hear back from the shop?