Start with basic troubleshooting:
- Swap the Slingboxes, does the issue follow the Slingbox?
- Swap the Ethernet to wireless adapters, does the issue follow the adapter?
- If not either of the above, does the issue follow the physical location (i.e. poor wireless performance)?
The fact that Slingbox #2 has choppy video inside the LAN is a strong indicator of an infrastructure issue.