Thanks @MattiasNYC I hope I get past it too, it’s driving me crazy.
Yes, latest firmware on SSDs.
I’m in touch with Gigabyte but nothing they’ve suggested has worked so far, including on which TB port to use (there are 2). But I haven’t ruled out the Quantum interface being the problem. This is because I also got problems with playback dropouts with the same project files with using my MacBook Pro with the interface.
I will try with my USB 2.0 interface tomorrow and see what happens.
Could my project files just be corrupt? Is that even a thing?
In the last month, I’ve worked on 2 projects, but used an iMac, MacBook Pro and now PC. Three different drives and folder setups etc. Perhaps Cubase doesn’t like the fact it has been moved about and on different SSDs.
With one of the projects, I get terrible disk cache spikes when cycling a part of one of the projects. The audio seems like it can’t keep up. It misses a beat and doesn’t quite restart at the right place when the playback gets to the beginning of the cycle. However, when I widen the cycle to encompass more music, the disk spikes go.
Disabling BT and network card did nothing either.
So tomorrow, I will try using a different audio interface and having a look at HWinfo64. Thanks for that suggestion.
Oh, yes, I get the Apple thing and agree. Which is why I really wanted to begin going down the PC route and have the possibility to upgrade parts and just have a really powerful system for all kinds of projects, including my tv/film music orchestral tracks, which is heavy on resources. I guess that is why, as you say, it is up to us to sort things out. I’m really enjoying the freedom and power of the system. Just need to sort this out 
Will update.