It could also be a problem of to much pressure...

Remove IHS and mount pot is often working fine for some time but after a few times benching there could be some problems coz there is to much pressure on core (some bga solder balls of the gpu core could be broken).

That is maybe the reason why a quite thick copper plate between pot and GPU HS and it worked well?

I heared from same issues a time ago (GTX280 - i think Shammy was talking about that) and that some cards worked really bad after a few times benching (bga solder balls of the gpu core had broken and some spacers to apply pressure at the right spots helped to solve that problem).

And also what SF3D is saying is right too - without HS you will easy run into some cold related issues and need really good TIM