I had a similar experience in my Ford Explorer.. I was on the freeway with the petal on the floor passing a few folks. When I went to slow down for an exit, the accelerator stuck - it stuck on the floor mat. It may seem silly, but it may also be a place to check especially if your mats are an aftermarket type with any treads or grooves on them. Just a thought.
Or it could be something mentioned above.
Maybe this helps.
1988 300E 90k miles