Verification is false (sometimes)?

[Bulmer]

I have been using your ipn listener for quite a while with no problems (thanks Mike) and have just recently found that I get intermittent false results for $verified = $ipn->verifyIPN() , even though when I resend the IPN post from Paypal they go through ok. Any ideas? The only possible problem I can think of could be using $ipn->usePHPCerts();//leave it up to php-curl as I previously had problems with local certs.