i've had my CDJ900 a couple of months now and i keep getting the same two issues both are intermittent and all in HID mode using Serato.
when you first turn the units on and start playing the decks will return to choose deck 1 or 2 screen, this normally happens within the first 10-15 minutes
if during a mix you need to feather one of the decks back to bring the track into time it will actually nudge forward 0.5-1 beats or if you need to do it forwards it will jump forward up to 2 beats
im using a Macbook pro running snow leopard, both decks are connected via a powered Belkin usb hub, firmware 4.05 installed and using Serato 2.3.1
i've also posted this on the Serato Forum