Playback glitchy sample rate conflict HELP !!!

Hi

May I ask for some help

Working on a track and keep getting terrible slow glitchy playback, then plays all OK then sudenly glitchy, pops and slowly grinds to a halt again

I get the warning:

"Sample Rate 36,489 recognised. (though the figure changes every time it grinds to a halt)

Check conflict between Logic Pro X and external device."

Sample rate between Focusrite interface and Logic X both set at 44.1k, though some of the vocal steams I recived are 24bit but are 44.1k sample rate, I don't think bit rate makes any difference like it used to in Logic V 2.5 onward to when Apple took over & all Logic Pro now handles up to 32bit floating etc ?

Anyhow system no way overloaded, but have increased the buffer, also frozen all tracks, also tried internal Mac audio output instead of Focusrite but same result???

No further external devices connected, only happens with this session, its killing me lol

Mac Pro Late 2013 trash can 3.7 GHz Quad-Core Intel Xeon E5

16GB Ram

Mojave OS

Latest Logic

Focusrite Claret Thunderbolt Interface