Had a question for you.
Can you let me know what colored wires are currently connected in the back of the Slingbox? If you have the yellow wire connected, the Slingbox may be setup for Composite wires (Yellow video, Red, White audio), this is standard definition.
If this is the case, unplug the yellow wire and reconfigure the Slingbox for Component (Red, Green, Blue video with Red, White audio).
If this is not the case, some cable boxes have a setting inside them to manually adjust the video resolution. The Slingbox just mirrors the resolution that is sent from the cable box. Try adjusting the setting inside the cable box.
Please let me know if this helps!!!
Sling Media Moderation Team