My mom's old '83 Caprice used to do the exact same thing!!! We'd be driving along and then that damn QuadraJUNK would just go WOT without warning other than the pedal going out from underneath your foot.
One time my brother was riding with us, and he just freaked OUT! He was screaming, "turn it off, turn it off!!!" My mum took the key out and put it right in front of his face and said, "it IS off!!!" Hehehe, funny in a boat that'll scoot.
Anyhoo, the problem was the accelerator pump arm and/or the secondary linkage. I suggest you do the same thing. Good luck!
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch