Read post #18 here. That is the solution to these problems.

>>so it seems we need to revisit the following:
setInterval("alterText();", 110);

The problem is NOT how often you call alterText(). The problem is that at some point, alterText executes:
document.getElementById("n"+letter).style.visibility="visible";

where
document.getElementById("n"+letter) is evaluated first. For a value of letter whereby there is a non-existent id, null is returned. So basically at the end, you are trying to set the style property on a null object. Hence the error.


Again, post #18 on this thread addresses this issue.