Gotta investigate this further, but this is what seems to work so far:

When booting, manually add
noapic pci=nomsi
to the default GRUB entry.

2 days in a row that this trick results in flawless performance.

Why not add it to the GRUB menu once and for all and be done with it, you may ask? Because I remember trying it and that it only worked after the first reboot, never on cold starts.

So I'm gonna monitor how interrupts are flipped around during the coming week and report on my findings.