Actually I've found that most games use both TCP and UDP, though there are usually more UDP ports that are required to be open.
I think that it may be your connection if you're getting ping spikes and a short disconnect... Though it could be certain packets of information are being rejected (I guess...
). You have a router, right? For ONLY A SHORT PERIOD OF TIME, check the DMZ for your computer's network address. Play online a bit (not too long) and see if you still experience the same problems. If so, it is your connection. If not, you need to configure some ports that should be opened.