So this is getting to me as well:
Responding to PM's seems to be a crapshoot, often times I have to refresh the page or retry several times to send a response via Private Message. I've found copying the text, closing the page and trying again sometimes helps.
Just today, when sending a response it would load for a long time and have "0" as the confirmation; and by confirmation I mean non-sending of the message ;(
This affects me both under Win7/ IE 8 and OSX/ Safari