How is the PC connected? Wired or wireless, and does this connection ever change while troubleshooting? If wireless, what card/usb? If wired, try switching to 5GHz for use with the ATV.
It could be 40MHz mode causing interference with other 2.4GHz devices and lowering throughput below an Apple set threshold for local content, internet content might have a more lax threshold. With 2.4GHz N, you should always run in 20MHz mode to cut down on interference, 40MHz mode should be reserved for use with 5GHz only. Too bad the ATV 2 is only single stream and not MIMO.