Onclick Handler Firing Multiple Times When It Shouldn't?

I have a web page that tracks clicks on certain hyperlinks. I am using
attachEvent() to attach to the document onClick handler, for IE browsers.
It works fine, except that for about 1 out of every 6 clicks, I get 2 to 4
click events for a single click. I know this because each time the
hyperlink is clicked, I write a record to a MySQL database. I write the
record to the database by setting the SRC property of an IFRAME on the page,
to a tracking script.

If I look at the database, for 1 out of every 6 clicks, I see 2 to 4 records
for the same click event (occassionally as many as 6 records). They are at
least 1 second and at most 7 seconds apart from each other.