1995 Mercury Villager No Acceleration and Power Loss

I recently purchased a 1995 Mercury Villager. Shortly after purchasing this vehicle, I noticed that it was incredibly slow to accelerate while the AC was on. I assumed this was not an issue due to the age of the vehicle and that normally an AC does cause a bit of power loss. Now, however, it is to the point that it will not accelerate from a stop with the AC off or on, even if I give it a good deal of gas, until the car has been on for a few minutes and is still sluggish at best from take off. It is also making backfire noises while decelerating. I am unsure as to what is causing this but would love to know as I do enjoy the vehicle. Please Help!