Ping requests gives replies but times out

Hi guys,
I am hoping someone can help. I have a problem with my network. It is up but at times tends to go down for a second, sometimes a minute. I dont know whats changed, but this started on Monday. When i ping my servers and pabx, i get replies however timeouts every now and then. Please refer to my attached picture.
Any help will be much appreciated.
Thanks
Donovan