|I would try another port on the upstream switch. |
Also, I would use a crossover cable when interconnecting two switches. Yes, I'm know it isn't absolutely necessary if you have auto MDIX capable switches but it's still a recommended practice and removes one more thing to check when troubleshooting issues like this.
In fact, I would wait on the crossover until you've tried a different port (it's always a good idea to do one troubleshooting step at a time). If the problem persists, change to a crossover cable and see if the issue goes away.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.