Problems with Mac Mini every time Backup starts

We have 2 Macs in the house, a Mini and a MacBook Pro; an iPhone, ipod touch, a Sony and HP Laptop, and an old Windows NT machine for storage. The only machines using the Time Capsule for backup are the Mini and the MacBook. The Mini is wired via Cat5 to the TC, wireless is turned off. The MacBook and HP, Sony, iPhone, and iPod access the inet wirelessly via the TC (so I have to have dual band rather than N exclusively) but both windows machines are rarely used and usually powered down. The NT server is wired into the router on the the TC to access the inet but there is usually very little traffic to the NT machine. I have an Airport Express upstairs that is also hard wired to the TC to extend the network.

So the problem: whenever backup starts on the mini the machine essentially stops all other processes. When I try to stop the backup I get the pin wheel and eventually endu up trying a shutdown which either takes forever or is preempted by my impatiently just turning the Mini off. Backup seems to start very frequently and I have not found a way to limit backup times to evenings. Why is backup bringing the mini to its knees and is there a way to give backup a window within which to operate?

Thanks!