Bought a untested 1080ti SC2 Hybrid off eBay, buyer had returned it due to overheating issues, bought it on a whim with no experience of this kind of thing but needed a cheap stop gap until I could afford to upgrade my card. 

Card was running at over 90, I suspected it was the pump. The pump seems to be working but I feel it has dried up inside, (repaste didn't work). Didn't feel confident with refilling a AIO because while it can be done it shouldn't really.

Bought a used 980ti heatsink, screwed it on, chucked a 12cm case fan on it and tied to together with garden wire. In the most absolutely demanding games it now hits about 85 tops but doesn't throttle, Furmark can make it peak about 90 still though.

I had to remove the copper plate but left the thermal pads in place hoping that'll do enough for the VRAMS, I've also left the other active cooling in place for the other end.

Seems to be working fine, no more throttling.

Thought it might be of some interest to anyone with a old card, wish I'd made a video of it to be honest because I could find little resources to whether this could be done easily for this specific card.

Irony is i've fixed it the day I've got a 7800xt finally coming, but I feel I've beat this challenge because it was annoying me I was sure the card itself was fine.

pics here: