Okay, so when you hit 31 songs cached, songs stop playing. Quick fix is to delete some files in your cache directory:
On Vista/7:
C:\Users\[your user name]\AppData\Local\GrooveWalrus
There should be a bunch of files like "7273ae54ab6b391a7e32bb3...", delete a bunch of them and you should be good to go again. Fixes coming out soon.