I was getting this yesterday as well. Not quite as frequently as you were, but occassionally. I've also had major memory usage where I have to quit out of my browser (Firefox) and then continue by reloading the page, until memory used by Firefox grows again.
I'm a fan of Google Docs in concept, but the implementation of the spreadsheets at least leaves a lot to be desired in terms of reliability and performance.
Check your memory usage, and completely exit out of your browser and restart to see if that temporarily fixes the issue.
I'm also told that Chrome seems to be snappier with Google Docs.