There are a number of things that can cause that, but the most significant factor in viewing quality is the available upload speed at the Slingbox.
Here's a rough test: if it looks good over the LAN where the Slingbox is at but bites when remote viewing, it's probably the upload speed. That's easy to check if you're using a laptop and go over for a visit.
Here's a more refined test: on your Mac's SlingPlayer, try holding Option and go to View > Show Statistics Window. (Option toggles a hidden menu item) The information in there might shed some light on what's going on. If the reported live bitrates are low (say, below 600 kbps) or wildly jumping, you've likely found your problem. Fluctuations (+/- 150 kbps or so) is normal. Also bear in mind that less complex, low motion video requires less bandwidth. If you're watching hockey and it goes to a tranquil "don't text and drive" commercial with only words, the bitrate will drop dramatically.
Another suggestion is to play with your streaming settings in Slingbox > Preferences. Take a look at the Video and Encoding tabs. I doubt changing things here will help much (or at all), but it won't hurt anything. If you paint yourself in a corner, just turn the automatic stuff back on the way it was.
Hope this helps,