SAFARI Re-Size

I am having a problem with Safari. When I am reading in the browser, the screen suddenly jumps to a new, very large size, too large to view on the screen. Each time this happens, I have to go to View, and reset to Actual size, which is really too small for my eyesight. Then, several seconds or minutes later, it does it again. I am not even touching the keyboard when this happens. How do I stop it?