Switching between two 970's and overclocking

[y3llowduk]

Hi All, I've been donated two 970's. I have been overclocking them both to see which one to keep and which one to sell. I haven't done any overclocking since a HD6850 years and years ago soooooo excuse me if this is a noob moment

 

Is it a no no to be swapping between the two 970's without changing anything else e.g. reinstalling drivers etc.? I've been experiencing a weird issue whereby since overclocking the Zotac 970 (it did extremely well!), trying the other ASUS 970, then swapping back in the Zotac, the Zotac now seems to hit some weird power limit. It can't even get close to the clocks it was at before. See screenshot with it pegged at 100% usage. I've tried different 6-pin connectors, tried it in the other motherboard slot etc. Something else which is weird is that the cards refuse to SLI no matter what I do.... seen some old threads about others with the same issue, a lot mentioning Zotac. Wonder if there's some difference in the cards which means swapping them gets stuff all confused? Don't care about the SLI not working, just thought it was curious.

[Bananasplit_00]

Swapping GPUs can sometimes cause issues, even if it's the same GPU so if you encounter any I would DDU and install latest drivers again