An obvious problem is the port you are using. If FrostWire is also configured
to use port 6346 then your firewall might be preferring that program right now.
Give BearShare a different port by changing the BearShare rule (in the firewall)
to any new port between 1024 and 65535, then in BearShare go to
Setup/Connection and change the 6346 at the bottom to your new port.
Finally, if you have a router add a new port forwarding rule in your router's setup.

To reduce the load on your connection, you can go into Setup/Service and
select "Never become an ultrapeer".

Perhaps your connection can't keep up to it in ultrapeer mode. Lets try the
obvious first fix and raise the number of connections you can open at a time:
http://www.lvllord.de/?lang=en&url=4226patch/faq

If it's still not working maybe it's the new BearShare that broke, your best bet
is to roll back to 5.1.0.beta 25 and use the BearStart utility to start it.