The general consensus is that this is caused by the BOV, which can stick or plug. This appears to be especially true of the 2G BOV, although some 1G owners have had this problem as well.
Alexander Kowalski's Jan 27, 1999 fix to his off-throttle stumble went like this (edited for presentation):
"I took the BOV off a today for a closer look. I found some RTV plugging up a 3 mm diameter hole at the base of the BOV. This hole appears to be part of a passage in the BOV casting that travels straight up to the top. I am assuming it is some sort of return relief passage.
Not only did clearing the RTV solve my off throttle stumble, my BOV no longer sounds like a loud bird shriek between shifts. Its more like the soft 'phfft' sound I have associated with my two previous BOVs. Darn, other than scaring the heck out of my wife I really liked that sound."
Cleaning the BOV, replacing it or upgrading to a 1G unit should solve the problem. Owners of adjustable BOVs report that setting the BOV too tightly will cause this same problem, so a quick adjustment may be in order.
Owners who are having the problem with the engine RPM dropping abnormally low after letting off the throttle may be having problems with the speed sensor or idle switch on their cars. If one of these is malfunctioning the ECU may not realize the throttle is at idle until the engine RPM drops below the normal idle speed.