Finally found someone with a similar problem. I have a tach issue similar to yours, but I have no power seats. On mine the tach drops and the ECM derates the engine to where it will not run correctly. It also has a miss at idle and runs kinda rough. I have checked all grounds I can find and to no avail. I have slaved a different pump, different engine wire harness, and different ECM, all to no avail. I think it is an electrical issue but cant find it. Also has the dealer stumped. Have you found anything on yours yet.