+1 on the fuel injector wires breaking down, should I be surprised the shop didn't check this when they changed,not really. Aso check your connections to your speedometer sensor and plug. Over the years I've see some strange things on these machines.

 

Fill the gas tank if that helps then the problem returns as the fuel level drops it may be the in tank fuel line has abraded and developed a leak

 

Good luck with it, I know how frustrating ican be trouble shooting injected engines. Being as it idles fine but the issue shows up under load I have to wonder about the TPS and the MAP sensors, it would be worth checking out too.

I think 06 was the only year plagued with the 8 degree injectors and it was just the early models but I could be all wet on that.

 

If it is setting the code, then the ECM is seeing a problem with the injector circuit. There should be power to one of the injector wires with the ign. on. The ECM grounds the other wire to pulse the injector. Test for 12 volts at the one wire using a headlight bulb to give it a good load test. If it lights the bulb, then you need to go after the other wire from the injector to the ECM. Make sure the female pins fit tight on the male pins at the connector. Loose pin fit will give you intermittent problems. Look for any arias where the wires could chafe. If you can find a flow chart for that code, try to follow it. If its not the connector pin fit or the wires, It might be the ECM itself. The fuelpak might have smoked it.

I hope you find the problem.