What is the size of your page file (virtual memory) in Windows? Claymore (and some other miners as well) require a fairly high page file in order to function reliably. If you haven't already, try setting it to the total of your VRAM across all GPUs.
+ Merit.
This is a great tip, I'd never realized that. So matching the VRAM will help maintain the hashrate? I've watched so many videos on mining and nobody has ever mentioned the importance of the page file. Gonna adjust it now.