Your first goal is to determine where the packet loss is happening. The areas of possibility are 1. Your pc, 2. Internet modem, router and home network, 3. Internet connection and 4. Cod4 server/network. You need to eliminate each area until the problem is found.
1. To eliminate the pc, try using a different pc on your home network.
2. To eliminate the home network, plug pc directly into Internet modem.
3. To eliminate Internet connection, start this packet loss test and have it running while on Cod4.
If a different pc has the same trouble and you experience the same problem when plugged directly in the modem and the packet loss test finds no trouble, then the problem is likely with Cod4 or the issue isn't packet loss.