Issue: Delayed PM notifications

I don't know how many others can replicate this, but I'm subscribed to instant email notification for the PMs and since past week they're working on and off. Sometimes I get the notification immediately and sometimes I won't get any for duration of 1-5hrs and these notifications come next day randomly through out the day. Firstly it's very inconvenient when I don't get email notification because I only check site when I get email, plus getting notification while there's actually no PM throws off false sense of alarm. I've not had this issue up until now, and the PMs that are missed do not fall in timeline when forum is usually backing up database. (Even if it did, I've got PM notifications just fine in past during back up too). It's not my internet as far as I can tell because I do get some of the notifications, and subscription emails from some other forums I'm on (ipb or vbulletin based) are working just fine.

Any help in resolving the issue would be greatly appreciated.