XPS 8900 Periodic focus switch

I have a XPS 8900 with Windows 10 and every few hours the focus will switch from my current program to the previous program I used. This has been happening since I purchased the PC, but I am finally annoyed enough with this to try to find out what is going on. This happens whether I am at the keyboard or not.

I have a similar problem with an older XPS using Windows 8 although in that case it doesn't switch to a prior program, but just loses focus on my current program, then it automatically returns focus a few seconds later. Then a few seconds later it repeats the loss and return of focus. Then nothing happens again for a few hours. I made a serious effort to solve this myself, with the steps listed below.

As recommended by Microsoft I disabled all non-Microsoft services through MSCONFIG. When that didn't help I removed all startup programs that weren't essential, including my anti-virus. I then went in to task scheduler looking for programs that were scheduled at the approximate interval and time when this happened. I disabled those and the focus loss still occurred.

I've seen reports of device drivers causing this problem and since both are Dell PCs, I am hoping they know if devices they include with their PCs sometimes cause problems like this.

Henry