Nissan Repair: 96 Maxima VQ auto rmp drop drastically when taking foot of gas, throttle position sensor, vacuum hoses

Question
Dear    My Maxima has the above symptom. Normally, when you drive for instance at 60 mph the rpm should be somewhere between lets say around 1800-2000 rpm, and when you take off the foot from the gas, it should then drop down relatively to the car's speed which may be 1200-1500 assumably. In my case, from time to time, for 60 mph, once I take the foot of the gas the rpm would drop all the way down to 400-500 rpm and may jump up a bit to 700-1000 rpm and stays there, though the car is still cruising with some speed. This does not happen all the time. For a 20 mile ride, it would happen 2-3 times, lasting about 10-45 seconds each throughout the ride. I've been trying to solve the problem but nothing seems to work. The car hasn't been stalled so far. seems like it happens more when hot.   For your info: The air filter is clean. The throttle body and idle air control solenoid had also been cleaned. According to the repair shop all the vacuum hoses are good and in place. All the spark plugs and coils are in good condition. The fuel injectors, injector rail, fuel filter and the hoses right from the fuel tank to rail had also been changed. The car once had a backfire and the air filter, the one that is attached to the air mass sensor was cracked. I glued it up and still using it. The car idles seems fine at 700-750 rpm without load, 800-850 with loads. Sometimes drops to less than 500 when in "D" gear. The symptom always happens with or without engine load. Thank you in advance for your help.

Answer
Art,

Either you have a defective throttle position sensor or the idle air control valve is sticking.  Yes, they could have cleaned the idle air control valve but it could be weak and starting to fail.  From all the things you have done and from the symptoms the above is what I have experienced.