I was having a similar problem on 2 of my rigs, after a lot of trawling the web I found something that helped me.

Try shutting down your mint wallet and running a different wallet/qt, any should do -I used cinnicoins wallet myself. I didn't have to sync it up, just get it running, leave it till it started to connect and then shut it down and run my mint wallet after that. It was like qt was stuck trying to do something and the only thing that would sort it out was that.

It works every time on my other pcs, if I don't do it I get maddening app hangs and a eventually a crash to desktop. Even if i wiped the wallet and installed a fresh one it was still happening.

It happened on 2 of my 4 rigs and I use the same thing to fix both of them.

Have you had any crashes/unexpected shutdowns on those pcs?