I expect a proper(inline) UPS would stop it.
Not really - if your PC goes from 30-40W (usual x86 desktop idle) to 120W that will show up on the other side of the UPS. It is current which is being pulse modulated here, not voltage.
The only thing which a normal UPS may do in this scenario is to decrease the effective bit rate a little bit.
You need a rather special power supply to defend against this - one with a constant current draw which discards all "unused" power by heating the air, water, charging - whatever. Normal UPSes do not do that.