Lots of people having this problem including me. See: https://community.sling.com/thread/2282?tstart=0
As far as I can tell Sling has not acknowledged this problem. The only fix is to keep resetting the box when this happens. This fix is only temporary and not ideal because you are likely not next to the slingbox if you are trying to access it remotey. Or downgrade to an earlier firmware which doesn't have this problem.
Chocobolee is correct.
I had the same problem, twice. This fall I upgraded my FW on my HD Pro to 1.4.060 from 1.2.100 and started having problems similar to yours, making my Sling useless. I rolled back the FW to 1.3.170 and problems were solved.
I purchased the new iPad app, and with Sling's promise of new and improved streaming in FW 2.1.080, and being told my Sling customer service I had to upgrade the firmware to use the iPad app, I upgraded the FW to 2.1.080.
Same freezing issues.
This is not a client OS issue as I am able to replicate the problem on Mac OS 10.6.5 and latest updates of WIndows 7, as well as iOS 4.2 on the iPad.
I have a feeling that Sling's response will be "use the web portal". I don't want that, I don't want Sling to have access to my device (they are blocked by my firewall.
I have not tried to use the web portal, nor will I try it as I use a VPN to connect to my Sling HD Pro.
While I wish Sling would fix this video problem, it has been throuroughly documented in the forums for over 6 months now, I doubt they will.
On a side note, while the firmware rollbacks fix my problem, you could also look into bad capacitors which when replaced by the user (Sling why don't you fix this problem too?) seems to help them. Hopefully Sling doesn't delete this post, here is an external link to a very detailed instruction on how to replace your capacitors (yes it voids the warranty, but since Sling won't support you anyway what do you have to lose?). Placeshiftingenthusiasts have very good forums for Sling as well.
Good luck, it is a great product, when it works.