Why does my car die when it slows down/stops?

It literally dies every time I either brake or let up off the gas, but starts back up fine. I recently changed 3 spark plugs (1 had stuff in the way and I was strapped for time). I plan on getting to the rest tomorrow. What could cause this?

I'll also add that, when it slows down, all the lights get dim, but when I accelerate, the lights go back to normal

The car is a 2004 ford explorer eddie bauer 4.6L

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