I'm on Logic so its not that. I tested for a few hours this morning using different configurations and found this interesting. I booted up a fresh logic project and added one instance of avenger and left it idle with the Init preset loaded. The intermittent 100% cpu spikes continued, as I expected. After 5 minutes I added a second instance and the idle spikes stopped. Looking at the cpu history graph, you can see the efficiency cores were working overtime whilst I only had the first instance added, then they immediately dropped when I added a second.
FWIW, I dont have efficiency cores selected to be used in logics CPU settings, and as far as I'm aware, logic ignores efficiency cores completely anyway (even though it's listed in the options).
To anyone else getting idle spikes, maybe you could try this yourself and report back here. I'll send my findings to support, but hopefully it's a step towards working on what the f**k is going on.

