That means that you have sniffed alot more than what your network can handle, or it can be that you have a slower network card in you computer like 10mbps where the rest of the network runs at 100mbps or faster and now you try to sniff using APR poisoning, so what you must do is set your cain and abel to only sniff between certain computers and the gateway and not all the computers and the gateway, the whole network will stop to respond to any service because APR poisoning generates alot of extra traffic (ALOT!) and thus all the requests on the network will timeout! All you can do is to stop sniffing for a while and then start again after a while just to give the network some time to breathe again. DoS stands for Denial of Service and APR is a method of hijacking IP traffic, read the help file of cain and abel to get more info on how to use it and what it does, you can also read my article on DoS attacks.