Try not using the Resume/Find Sources function on all your files at once. Also consider doing this: Tip: Purge the Downloads Queue!.

A sudden push of traffic such as resume function or having lots of incompletes attempting to connect at once may cause this. Also, disable Ultrapeer capabilities under Tools>Options>Speed. And disable OOB searching in same window. Also disable iTunes support/sharing. See if this helps.

I can understand UPnP failing or being unreliable but thought port forwarding would be more reliable. What you should normally do when port forwarding is do a separate port forward for each of tcp & udp. I was told last year by someone who uses a different p2p technology that the router will behave better when this is done.