This article: Network Speed Requirements for SlingPlayer for iPad provides some good information. It's typically the upload speed that impacts buffering.
If you have a moment, test your speed on speedtest.net and see if you are meeting requirements.
We had this problem on our iPad also - before we got the Slingbox
We experienced terrible buffering problem when we tried to watch Fios Mobilte TV on the iPad - it really was unwatchable
To deal with this problem, we upgraded our Verizon router and increased our bandwidth on our home network
Then we also got the Slingbox. Everything working as it should - problem solved