[phd]

To diagnose the situation, you can try shutting down each and every wifi devices while you play.
If the problem occurs anyway, then it is not the problem.
If not, then it might be because too much is happening on your internet at the same time.

You can try changing some network configs
For example :
seta cl_packetdup 3
seta cl_maxpackets 125
(I picked those from my autoexec, I am not sure those settings still exists in 4.2)

You can try using an RJ-45 cable. See whether the problem come from the WIFI or not.

Try going to a friends house with your computer/laptop and play there. If the problem still occurs, then the problem is located on your computer.
Have a friend come to your place and play from your network. If the problem occur, then the problem is from your network.

Again, an old router will work, but slowly becomes more and more unreliable.

Good luck!