Logic Pro X Logic & iMac Firewire problems - Distortion, Feedback & Glitches


I've been experiencing a problem with Logic using either my MOTU 828Mk11, or my Mackie Onyx1620i.

These sound cards work perfectly until I open logic.
From there I have random feedback esq distortions on stereo playback, and random digital sending glitches through the system, even when not using logic.
I bought the firewire desk to aid this problem but it persists.
I really want to avoid re-installing my Mac in full as I have stuff that runs on Mojave that I don't want to loose.
It feels like there is a problem with the IMACs Firewire audio.

SO I'm quite baffled as to what could be causing this - has been going on for a while now, just been working around it, not ideal.

Any suggestion appreciated, many thanks

My system is -
IMAC 27" late 2015 / 32Gb
MacOS Mojave 10.14.6
Motu 828Mk11
Mackie Onyx 1620i.