Wordpress EC2 instance stops working regularly

Hello.

What EC2 instance type are you using?
For example, in the case of T-based instance types such as T2.micro, once the CPU burst credits are used up, the performance will drop to the baseline throughput, so this may be related.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/burstable-performance-instances.html

Please also check the EC2 load status and system logs.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/troubleshoot-unreachable-instance.html#instance-console-console-output