P1507 idle control system malfunction

Occasionally, when I slow down from a high
speed after driving for a while, my check
engine light comes on and the car starts
driving roughly and losing power. If I pull
over and wait 5 or 10 minutes, the car starts
easily and will drive with normal
acceleration, although the engine will not
sound as "smooth" as usual and the check
engine light will stay on for a while --
sometimes only a few hours, sometimes
longer. Once the check engine light goes off
the car runs normally. This has happened a
few times each of the past few years. I had
the neutral safety switch replaced the first
time it happened. Another time I had it
towed to the garage but they were unable to
replicate the problem and their diagnostic
check didn't find anything (the computer
read re: the check engine light was p1507).
The last time was worse because I couldn't
pull over on a hill and I had the gas pedal
floored and was only going about 25 mph.
There was a slight burning smell when I
finally parked. The next time I drove the car
(5 days later) it was fine, and the check
engine light hasn't come on since, but I need
to figure out what is going on. Any advice
would be greatly appreciated.