Looking at the error you are getting, which says "Script invoked too many times per second for this Google user account", I don't think adding a sleep will help. It is because the number of times your script is called is the point of contention. Try reducing the number of calls to your script in the spreadsheet (basically reduce the number of cells with formulas invoking the script). If your script has cyclic calls, try removing / reducing them.
You might get a better answer if you can provide some more information on how many cells call your script and perhaps some relevant code from your script itself.