Thanks for the prompt response.
Overnight, I decided to test whether FUSE was causing the issue. I remapped /mnt/user/downloads to /mnt/cache_patriot/downloads - no joy. Not FUSE related.
After reading your response I remapped downloads & unpacking (.../usenet/incomplete & .../usenet/complete) to RAM (via Unraid's /tmp/... folder). My logic: I wanted to identify whether it was a CPU problem (which I doubted) or IO issue (which, admittedly, I also doubted) - problem solved! Downloading, unpacking and or repairing in RAM resolved the yo-yo problem.
I have to admit I was surprised, my current Cache Pool arrangements is:
Cache_nvme: 2x Raid1 NVME: For Docker, VMs, etc
Cache_patriot: 1x Cheap Patriot SSD: For downloads, transcoding, and anything with high IO
Cache_ssd: 2x Raid1 SSD: general Cache for data requiring redundancy
The yo-yo occurred on both the patriot & raid1-ssd cache pools. It didn't occur straight away, for between 20m and ~1h of sustained 88MBs downloading, SabNZB was fine downloading and unpacking files. Randomly, it would start, and occur regularly, not going away until a restart. After the restart, the cycle would start again. This pattern of behaviour made me think it was a strange i) Unraid issue, ii) Sabnzb issue, and or iii) a combination of both. This thinking was compounded by the problem having only started occurring in the last ~2-3 months (it ran fine on the cache_ssd pool for a long time, when I first setup Unraid).
I'd love to continue downloading to RAM. But my server lives in a Jonsbo N3 which requires an mITX mobo, max RAM at this stage is 64G. This isn't sufficient for large nzb files.
I have moved my downloads folder to the nvme Cache Pool which works fine (just like RAM). This is probably not an ideal long term solution as the high IO is using up 2x NVMEs - the whole point of the Patriot SSD was to avoid this.
I may have to find a good SSD which can deal with the IO.
Thanks for your help. Excuse the length of this post, I wanted to share in the hope it helps others. This was driving me crazy!
PS. If anyone can recommend an SSD which works well with SabNZB, please shout out.