I've seen two cases of "unrequested acceleration", and both cases were
caused by rodent nests in the airbox. The little nuts and twgs these guys
were stowing in their new home were intermitently getting sucked down the
intake boot and lodging themselves in the IAC motor, preventing it from
closing completely. They would stay in there for a minute or so and
prevent the engine from idling under 2000, then dislodge themselves and
everything would return to normal until the next time.
Just something to think about.