Not Respounding status

Since I have downloaded 3.1.2 I occassionally get a "Not Responding" message in my status bar, especially when I have an attachment. The only way to get out of this is to shut down and restart Thunderbird but then I lose the message I was trying to send.

I will try going down one version since I didn't have this before but just wondering if this is normal or is something the matter. Anyone else having this issue???

Thanks.