This is definitely not jitter. Jitter is small timing differences in the data as sent to the DAC. Jitter would not cause what you are experiencing.

My guess is that for some reason the data stream is being interrupted completely or almost completely. This usually happens when the computer has lots of other things to do, like burn a CD.

If it is happening when the computer is doing nothing else there is an issue with how resources in the machine are being used. I can't be of specific help as I know little of Macs other than that they are fine machines.