I think 80% on one core and 30% on the others is where the problem is — I also agree that decreasing the instances of plugins will help, but surely it’s only by decreasing the load.
My thoughts are that there is a vsti causing problems, maybe conflicting with another vsti, I may be wrong but what I would do is backup Project and then remove one Vst (all instances) then check how even the load is over the cores then replace it and remove another and see if there is a culprit, I’d start with third party ones but leave Kontakt until last and do steinberg ones before Kontakt.
Other thoughts are you may have reached the sound card drivers limit. But then I think why uneven across cores and come back to above idea.
I think your PC must have 4x 2Gb sticks plus 2Gb on the graphics card.
I have never seen a 3Gb stick and the other option of 2X 4Gb plus 2X 1GB = 10 Gb with a 980X just seems wrong, but is probably right.
Having bought a PC with the daddy of all processors I would definitely want Tri channel memory unless there were stability issues, I wonder if HP found configuration problems on the platform and had to settle for dual channel, might be worth asking them especially if you want to reconfigure, it might just mean 1 stick of 4Gb depending how they got to 10 Gb.
More Geek orientated is audio issues with core parking but that really is for the hardware section
Hope this helps.