Network Problem cause by looping cable

This is one of the most challenging problem i have to face. The campus network that i manage use 3Com 4400 switches. One day, all of the sudden the network becomes so damn slow, more packet loss and slow internet connection. Currently there was a network cable contractor who was doing some work on adding network node.

I try to use WireShark to find what going on in the network. Nothing found or to be exact i didn't understand at all what is displayed in the WireSHark. So i decide to check where were the cable contractor work on the day the problem occurs. I check the 3Com 4400 switch on the each place where the cable contractor work at. Pull in and out the cable from the switch. Luck on my side as when i pull out one cable, there were two port stop blinking. From there i am pretty sure this mess is cause by LOOPING CABLE.

I am not sure whether the STP are configured correctly in the switch. For more info go here.