Puzzling system delays
Every couple of days my 2015 MacBook Pro running Mojave 10.14.4 goes into a state where it simply pauses for 2-3 seconds. I can be typing and nothing comes out, but then in a couple of seconds everything I typed spits out on screen. I can drag a window and nothing happens, and suddenly the window jumps where I moved it.
A reboot fixes the problem temporarily and it comes back in a day or two.
I tried quitting every open program one by one, then every menubar app. Then I turned off Bluetooth. Then I turned off WiFi. Problem persisted.
According to iStat Menus, there's no system load at all and no network traffic (in KB/s).
I reset the SMC and NVRAM for grins and giggles. The problem returned.
Right now it's only pausing maybe every 5 minutes. Sometimes the pauses are every 30 seconds.
Any idea what this could be and how to diagnose?