2001 Mercury Villager Sputtering

My car has been sputtering when I go uphill for about a week. I thought that it was the plugs or the wires and intended to fix them this week. Yesterday, it began sputtering the entire time that I was in the car, even when idling. It finally got so bad that I could no longer gain momentum and had to be towed home. At the same time that this was happening my speedometer began going up and down wildly. What could have caused this? Is it my transmission or something to do with the computer?