I'm assuming multicore rendering didn't help then? Do you have any special error messages in console while playing while recording demos?
Anyways...
Good news #1: The problem is 100% definitely in the recording of the demo.
Good news #2: I'm pretty sure the cause is that something is interfering in the packet creation of the recorder which is causing the "jitter" effect and creating an essentially "inaccurate" demo. You can very clearly see this yourself by loading the demo, then sv_cheats 1 and watch it in host_timescale 0.1. You literally teleport during the jitters and during these intervals your crosshair movement even drops usually. Basically, the demo doesn't have information there and just moves you ahead to the next packet of information.
Bad news #1: I have no idea what is causing it, everything reasonable in-game doesn't seem to be the cause which means this is veering into the territory of:
* a weird bug with some weird cvar (and there's hundreds of these)
* weird threading issue that goes beyond the multicore rendering setting (dunno if you have a similar CPU/GPU to the OP or what),
* something odd with the specific time these recordings are happening breaking the recorder (this used to happen in CSGO, idk if they fixed it, if you recorded a demo after freeze time your demo would break in a manner very similar to this)
* your HDD sucks enough ass at writing small data chunks consistently (this is demo recording in a nutshell) and is causing this.
Bad news #2: It's even harder to figure this out because the .dem format has not been reverse engineered to completion to really dig deep into this. (this is for the best as far as cheating goes tho, if you could do this you could edit them and hide your hacks better)
Bad news #3: This also means these demos will be broken pretty much forever tho.