Need some ideas to track down crackling noise

I have an environment where there is a master patcher that holds a collection of [vst~] objects and a mixing console.

For each song that I play in a set list, I load another patcher that contains the desired audio routings, MIDI routings and so forth. A file is also loaded which is used by the master patcher to determine what VSTs are needed for the particular song and to set all the volume levels. My Macbook is connected by firewire to a MOTU 828Mk3 which itself is chained to another 828Mk3. The second MOTU gets its wordclock from the first MOTU.

I listen to my own mix (which includes all VSTs as well as several external keyboards that come in) on channels 7&8 (first MOTU) and 8 individual outputs go out on channels 31 through 38 (second MOTU)

This has been working perfectly until we started our tour a few days ago. At our first performance a bizarre crackling sound started and the song had to be completed with keyboards absent. During sound check the next day, I was able to reproduce this and discovered a couple of things
*) It is only happening in one (thank goodness) specific song
*) It's not continuous, once it starts happening, it only happens when I actually play notes
*) The crackling can be heard on my internal mix as well as on the 8 external audio outs
*) If I restart the dacs, which takes about 1/4 second, the problem immediately goes away. (For now, I have mapped a pedal to this restart so as soon as I hear the problem, I can tap the pedal and keep going)
*) I have the DSP window open, I am not seeing any sudden CPU utilization jumps when this happens so it doesn't seem to be an overload issue.

As far as I am aware, there is nothing particularly special about the song patcher where the problem arises so I'm having a lot of trouble trying to track this issue down. There isn't (for example) a VST being used that is never used in any other patch. It can't be a wordclock issue otherwise I would be having the problem in all songs.

I'm wondering (indeed hoping) that someone might have some insights into some other possible causes of this problem. We have another show in NYC tomorrow night and although I have quick hack now, I'd love to track this down so I can stay focused on the performance rather than waiting to hit the foot pedal!

I'm running Max 5.1.9

Thanks in advance