Initial thoughts...
1. Could be an unrelated network snagette. What time of day were you trying to play YouTube? Weekend/evening/holiday, all impact on connectivity.
2. AV software running? Is Norton running a setup scan? (don't use it myself, just guessing, in which case everything will return to normal when it has finished). anything working in the background? Something is sucking up your processor, or the network you are on is very busy. Do you have Windows update on Auto downloading stuff?
3. Try Right clicking the Taskbar and select TaskManager. Click the Processes tab, and click the CPU column to sort high to low. Might show something that is causing the stuttering video streaming.