Thanks for reaching out to us in the land of the Forums.
Based on what you have posted, it looks like the router is in good shape and the settings are good to go. If the actual streaming is going down to 40, then that can definitely interfere with the streaming that the Slingbox needs to work accordingly. We have included a link that will offer some steps in getting this resolved.
Streaming in SlingPlayer is slow and doen't recover
The Sling Moderation Team