Activity monitor stops updating or intermittently updates
I know of 2 options to help you. The first is to open Activity Monitor and click on View menu. The 3rd option on the menu is "Update Frequency". Clicking on "Update Frequency" gives you several options for how fast the screen is updating. Change that to a smaller number and see if that helps.

The second option is to use top or htop from the command. See how it looks. Activity Monitor is just a user interface for UNIX command top. By typing top on the command line, you are bypassing the user interface part.