Captcha PNG files are showing as 404 in Pingdom, Google Speed Insight etc

The captcha is working fine on my forms but sometimes (not always) when I do a Pingdom or GTMetrix speed test on a page, the page takes ages to load because the Captcha png file is … 404!

Whenever my page comes up with an ultra slow reading in any of the speed test tools it’s because a file like wp-content/uploads/wpcf7_captcha/1142593786.png … is not found.

When I go look in the folder the file is indeed not there.

Is this a problem because of W3 Cache interfering with how Really Simple Captcha stores and serves files? The problem is not consistent i.e. sometimes it happens and sometimes my page loads fine without any 404 for the png file.