I also dont believe this is a PayPal issue - there are lots of reports of different and odd behaviour from upgraded sites that are intermittent even on the same site.
Thanks Al, it maybe purely coincidental but i did not have any PP issues until i updated from 6.1.1 to 6.1.7 on Friday.
if i compare the IPN messages before and after the upgrade i don't see any difference they all say sent and no reattempts the url is the same