I ran into this same problem a week ago on my 2007 350zHR. Decided to clean my throttle bodys with no idea that idle problems were highly likely after everything was put back together. My ECU seems to accept the ecu, throttle, idle "pedal dance" re-learn procedures initially but when i turn my car off and start it again the same issue occurs. Cold start idle is around 2kRPM. Once it reaches operating temp it drops to about 1.5kRPM...then continues to surge 500-600RPM up and down. The hotter the engine coolant gets the faster the idle surges.
So far ive confirmed that there are no air leaks whatsoever....spark plugs, coil packs, MAF sensors and O2 sensors are all working properly. Im a bit skeptical on buying new throttle bodys or an ECU because ive read that sometimes changing these don't fix the issue. Ive also read that it may be because of a blown catalytic converter?....i was literally about to sell my car too which is why i cleaned the throttle bodys in the first place..but now i think i may be stuck with it for a while until i can fix this issue or just sell the car as is at a lower price -____-