The main principle behind PhotoFocus is RELAXATION. What you are describing is a straining of the eyes that requires significant recovery time...that indicates you are doing something radically wrong.
If you are sitting at a table, pull back a little and look down to your toes. If a book is on the table so that you can see the lower portion of the book, you will notice that it has a blip. Your eyes will be totally relaxed. You certainly can look at your toes for five minutes without blowing out your regular focus for 30 minutes...right? It should be no more strain than that!
My guess is that you are not diverging your gaze, but crossing your eyes. If you do you still see a blip, but you do it with strain rather than relaxation. I would recommend you not go for the blip...go back to the "Betty Edwards" technique: notice the four corners, an imaginary "X" and the white space (negative space) on the page. Better that than whatever you may be doing.
You might get someone to watch your eyes as you go into PhotoFocus to confirm my hypothesis.
