dedicated server having lag ?

hello i have got all my server running but we are getting lag at times where the zombies are running on the spot for 5sec or so at times and just playing are lagging at times

i cant work out what it is doing it as we have a good ish dedicated server just for what we want to do

the specs are
quad core intel xeon l5420 2.5ghz
4gb ddr3 ram
240ssd
370mb download and 172mb upload
http://i1095.photobucket.com/albums/...psf0e11ab0.jpg

as you can see from the photo the server is not overworking the cpu , ram or sdd

anyone got any idea's on what it can be ?