Hi there and welcome Fred
There can be a few causes for this issue. May I ask please, what have you already tried to do to resolve this?
1. Called Apple Care. (AC)
2. At AC's suggestion I shutdown, powered down and then restarted while holding down the shift/control and option keys in order to regain control over the cursor and my keyboard.
This appears not to be a permanent fix, because the problem reappears.
3. I have experimented with putting the iMac asleep and with shutting it down. Either way, sometimes it reawakens normally and sometimes not.
4. The articles I've found on the subject of interference aren't much help, because all the potential sources of interference—iPhones, wireless house phones, and other devices and environmental furnishings and conditions have not changed from before the problem emerged.
Please don't forget the following question:
QUESTION: Would switch to keyboard using a USB nano receiver instead of blue tooth eliminate the mysterious interference problem?
I saw the following question, but too early to answer yet as we need to rule out a number of things.
First, are you able to do the following?
Power off the mac
Then start the Mac and hold down the Command and R keys at startup.
Hold these keys until the Apple logo appears. After your computer finishes starting up, you should see a desktop with an OS X menu bar and an OS X Utilities window
choose Disk Utility from the Utilities window, or from the Installer menu if you're started from an installation disc.
Click the First Aid tab.
Click the disclosure triangle to the left of the hard drive icon to display the names of your hard disk volumes and partitions.
Select your Startup Disk (usually named "Macintosh HD").
Click Repair. Disk Utility checks and repairs the disk.
Let this complete and when it has, tell me if you still have the problem please.
Sorry for the delay in responding. By the time I got your restart and first aid suggestions, I had received two new keyboards and had also restarted using various techniques, including the one your suggestion.
One new KB was another bluetooth model from Apple (free under Apple Care.) The theory was that maybe the antenna is causing problems. The new KB seemed to be better, but did not eliminate the problem 100%.
The other KB was a corded Matias Mini Tactile Pro. With it the problem(s) have disappeared and my typing is more accurate and comfortable.
The curious thing is that my Apple Magic Mouse (bluetooth) has never had problems of any kind.