On the off-chance that someone else experiences this and finds this thread, I managed to find the cause of my problem and fix it so this may be of use.
It was basically caused by electromagnetic interference. There was something plugged into one of the neighbouring USB ports causing part of the issue, and the power lead to one of the (display) monitors was responsible for the rest.
I plugged the CI2+ USB lead into a front USB port where all adjacent ports were empty and then moved the interface itself as far away from the monitor as possible and the problem completely disappeared. Playback is now perfect.