I understand that you have troubles connecting the Slingbox to the network using the 40 ft long Ethernet cable. Using a very long Ethernet cable could caused network problems like the one you are experiencing, we recommend using a power or a wireless bridge instead. Sling Media offers the SlingLink TURBO for this scenarios, it is a power bridge that work really good, have a look at this:
Even though the Slingbox does not support using a Cat5 cable that long, probably, a Cat6 cable should not give you that type of issues since the performance of this cable is better than the Cat5's.
Hope this helps.