Irratic behaviour of the switch network

Question

we have more then 50 switches with more then 200 computers. suddenly the network has started behaving iratically. all the network gets jammed and stops accessing to the central switch.When one switch is removed from the network it starts working fine. but swich may be any of the 50 switch.wat may be reason. 
1) I could not renew the antivirus, is it possible due to virus.
2)My switches are more then 4 years old .Is the problem related to the old switches.
3) the correct method of detecting the problem?