The throttle pedal has been know to get caught with the carpet.
Check under the dash where the throttle pedal is,and see if the top portion of the carpet is curled down,or hanging down.Happenned to a few of my friends where the throttle stayed at WOT,and they had to shut their stangs off.There is a service bulletin on this issue i believe..
If that is not the issue,take the throttle body off,and give it a good cleanning with throttle body cleanner,sometimes they get gummed up pretty good.
Also make sure that the return springs aren't stretched,or not missing.
Good luck.
patman0911, the first time it had happened the pedal got stuck when it was in the WOT position. The next 5 or 6 times it happened around 3/4 throttle. The pedal does remain pinned down when it occurs, and at this point I frantically stomp downward on the pedal multiple times in an attempt to get it to spring back upwards. Thanks.