constant? or has it jsut been happening lately?
start by doing this...
set yoru cl_cmdrate to 120
set yoru cl_updaterate 15
and yoru rate to 11000
then move yrou updaterate up by 5
check yoru netgraph to see if you have any choke, loss, or MS(the number right next to yoru fps on the first line)
if none appear move it up another 5 and conitnue this until you get something
then lower the updaterate until your MS is at or under 10 most ofhte time...
that is about all you can do for yoru game...
otherthings that are more than likely the problem are you ISP or LAN