You answered your own question. If you are pushing the NVR past its limits then clearly you need to upgrade to a better one or just keep lower settings on your cameras.
I have had good luck with my SanDisk microsd card in my hikvision.
I would not recommend a 64mb card, which are still made for whatever reason. I have a 32Gb card and it will record continuously for just over 16 hours at full 1080p resolution.
I'm just starting on a security system.
As of now I am having trouble getting my Hikvision online. Came here to see if I can find a solution.
Thanks for having me.