The first thing that comes to mind is the fuel pressure regulator, but you already swapped that out ... the other classic reason is the intake manifold gasket is on the way out ... but if it was so bad that it had issues starting it should have issues at idle too ...
another thing to check is the EGR tube that exits right behind the throttle body, that the opening on the tube isn't plugged with carbon ...
Maybe dump in a bottle of that HEET stuff if there is water in the gas?