Actually, there is a way for frame rate to be impacted by the Internet connection. If you run other network programs such as iTunes, or any type of P2P program, the hits on the CPU to handle those problems network activity can drain resources from the game, causing the frame rate to drop.
This will also be reflected in the game's "Net Status", "Variance" line. If the line is bouncing about, then there is a good chance there are background processes, possibly network related, that could be taking the CPU away from the game for long periods of time.