Our MADIface XT is being used to record 16 channels at 96k/24bit through Reaper 5.978/64 on a 2012 Mac Pro running OS X 10.13.6.

There are two issues, one of which is the occasional glitch, despite a large buffer size and CPU performance well within limits. I tested four different USB 2 ports on the Mac, and tried usb 2 and usb 3 cables. All were problematic. We were using TotalMix 1.6 but I downgraded to 1.5 and rolled back the driver from 3.15 to 3.08, but it's still glitching.

Is this a known issue on High Sierra? My understanding was that older Apple hardware was very reliable.

The second problem is more unusual. It sounds like a clocking problem between the XT and Micstasy/XTC - every 10-20 minutes, a recurrent crackling sound begins, and occasionally develops into a repeating beeping sound. However, it is not audible on the recording on playback, nor through TotalMix at the time, only through Reaper's monitoring. The XT is successfully set as the master, clocking through SC fibre. LTC is being fed into one of the XT's analogue ins.

What's the problem here? Are the two issues related?