Had this same code but mine ended up being the heads hopefully that isnt the case with youres. If you are good with tools do some diagnosing. I checked everything and came down to the valves in the heads and now runs fine.
First if you have the metal covers that go on the wires take them off that solved my issue on another gp i have. Second if you have a junk near you go to it and pull as many sensors as you can from 3800 great for having as spares that is what i do helps eliminate problems without the cost.
Check your Vac lines spray around with carb cleaner.