Cisco novice here...have a Cisco 2960 connected to a Cisco 3550 via Fiber (along with a few other 2950s and 2960s).  This has been running fine for years....starting Monday morning, the fiber link to one 2960 just randomly shuts off.   Doesn't get disabled...just goes into a notconnect state.  If I do a shutdown, no shutdown on the interface it'll come back up again....and then eventually it will go down (sometimes minutes later, sometimes hours).  I've moved all other connections off that switch onto another one...so the only thing connected to the switch is the single fiber connection...link still goes down.  Does that sound like a faulty GBIC or fiber cable?   Is there any kind of hardware log or diagnostic that can be done or run on the switch to tell?