Audio Distortion in Logic X

Hi guys and gals,

HELP!!!! Having an issue with audio playback in Logic ProX. Its a session I've been mixing on, and it has been trouble free up until 4 days ago. I added a tamborine track, and during the tracking, the playback audio started glitching. It would start distorting across all tracks and busses. I changed my buffer setting to max, and no improvement. I changed my multithreading to 16 instead of automatic, and its fine. Then the issue reappeared last night when i was making final adjustments to the mix. I starts playing fine, then 15 seconds in the starts barely distorting, and the further it plays the worse it gets. Like a ring modulator into a bit crusher. Terrible. Any ideas.

Its a big session. Lots of Plugs. All audio tracks already frozen. Lots of busses with plugs. Plug compensation on all.

...aaaaand, I'm running all ten outs of my interface thru my summing box.

Rig specs: 2009 mac pro 8 core 2.66 nalahem. 64 gig of ram. Running El cap. 3 stock drives, and a glyph external 5TB firewire 800 audio drive. Focusrite Saffire 40 interface. Dual Displays. Stock vid card.

THANKS IN ADVANCE!!

John