So, I have a bone stock 2005 MCS with about 89k on the clock. I took a road trip last weekend and was headed up a long grade (mind you not all that steep, but very long). While going up, the car kept stuttering and stumbling and finally lost all power. I floored the throttle and nothing, when I let off the throttle the car would jump (pretty violently, like just dumping the clutch) back to life. It did this 3 times in the 7 or so mile stretch. Upon the last time it did this, it came back to life and had quite a bit more pep to it, and I drove the remaining 6 hours trip with little incidence. I also noted that my fuel mileage was noticeably better during this period.

I had noticed over the last few weeks that the car has lost some of its zing and fuel mileage had fallen off by a few MPGs, I attributed it to the fact that its time for a tune up of plugs and wires and such, but now I wonder if that is the case. I just replaced the Serpentine belt a two weeks ago. And have plans for other maintenance items, like plugs and fuel filter, in the coming weeks.

This afternoon the car has returned to its lethargic nature. Does anyone have any ideas as to what may have happened? I'm really at a loss as to where to start looking for answers.