Clone Stamp tool keeps Zooming

While using the Clone Stamp tool, it periodically zooms in. Sometimes it zooms very fast from 300% all the way up to 1600%, and I see a grid pattern on the screen. I work on a large files that require a lot of touchup. I often lose my place when I zoom out, and then need to reestablish where I was working. It's very annoying. Sometimes I can work for a while without it zooming. Sometimes it will zoom right away again. I use the Option key quite often to select an area for cloning. It seems to zoom when I click the Option key. The Option key is right below the Z on my keyboard. Z selects the Zoom tool. I thought I was accidentally touching the Z key at the same time I was touching the Option key, but I have been very careful not to do that and the problem persists. Can anyone help me with this problem?