more than likely its your cable provider, some are real !@()#*!@)#*)!@(#*'ers they can detect what ya doing and slow the connection down after 1/2 hour.
However this being said, its always possable it is a local thing to you, dont give up all hope, just beaware it might not be your end.
I would test it, by seeing if you can spot when it occurs (ie: 28mins 30mins 35 mins) and then disconnecting mirc, and reconnect just before that, and if it donest appear you can be pretty sure its your ISP.