Try this @@Mule.
Open console with the tilde button, it looks like this ~.
If it won't open go into player options and enable console, then repeat step one.
Once open type into it:
/cl_packetdup 2
(The data you send to the server from your pc is sent as a packet or packet of data. This command makes your pc send two identical packets of data so if one is lost a second is there to back it up).
Hit enter
Then while console is still open type:
/cl_maxpackets 100
(This is the maximum amount of packets you can send in cod4, the default is 30 so changing it to 100 is beneficial too).
Hit enter.
Close console buy pressing the ~ button again.
These two commands should help you.
Any dramas let me know