System refuses to accept input form keyboard or mouse
Intermittently my iMac doesn't respond to the keyboard or mouse for anywhere from 5 to 10 seconds. Characters I've typed may appear very slowly, or not at all, and clicking accomplishes nothing.
Sometimes this is accompanies by a beachball. After a while it clears up and goes back to normal, only to happen again, usually in less than an hour.
I am running 10.14.6 (Mojave) on a late 2015 iMac with a 4GHZ Intel Core i&, and have 32 GB of RAM installed.
Activity Monitor shows memory pressure as fine and neither Techtools nor Disk Utility suggests any problem with disks.
I am using an external SSD as amy system drive, because the SSD part of my hybrid seems to have problems, but have been doing this for quite some time, starting well before this issue arose.
iMac 27″, macOS 10.14