My car does this also. The nature of the drop is similar to that of when the car is warming up (if it is indeed the same issue). Seems like the PCM is testing the waters at 500, finds out they are rough, and then retreats back to safety that is 700 rpms. Eventually it will find a good idle rpm (lower than 700 and above 500) but that last bit has some trial and error.

Same thing or no? If it is then its normal to my knowledge.