Help with ancient system running PT 11 error message AAE -6085

[Rooter Tooter]

Hello Friends,
I keep running into this error message after about 1-2 minutes of recording:
“The audio device buffer underflowed. If this occurs frequently, try decreasing the "H/W Buffer Size” in the Playback Engine Panel or remove other devices from the audio FireWire bus. (AAE - 6085)

My system is admittedly a bit long in the the tooth:
2008 Mac Pro with 2 x 2.8 GHz Quad-Core Intel Xeon
Yosemite
16 GB of memory
1TB Seagate SATA hard drive 7200 rpm
Digi 003 rack (not the plus version). It’s been upgraded by Black Lion Audio and has been a great sounding interface for my needs.

Yes, it’s probably time for an upgrade BUT I’m trying to eek out another year or two on this bad boy :)
A little bit of background; I’ve been primarily making music outside a DAW environment for the last five or so years but I did upgrade from PT10 to PT11 a few years back and I changed the hard drive I record onto from a 500GB Seagate to a 1TB Seagate as I was running out of space about two years ago. But that’s about it with the major changes to my DAW over the last few years. Perhaps when I upgraded from PT10 to 11 I forgot to turn off some hot keys in the Mac preferences that interfere with PT?

I’m wondering if anyone has any tips to stop the error message - it happens even if I drop buffer way down to the lowest possible size. I tried turning off Bluetooth and it didn’t make a difference. Any tips would be greatly appreciated.