Apache2::Filter Intermittently Missing Injected String

I have a script that uses Apache2::Filter to filter the server response
output and inject a string into the HTML body. The script normally works
fine expect intermittently the output is missing the injected string. This
happens around 10% of the time. I verified that there is enough memory and
CPU available and tried playing with the buffer size, but to no avail.

The server is running:

Apache 2.2.17-2
Modperl 2.0.4-7

Any explanation for why the script fails 10% of the time?

Thanks
Chris