On both my desktop (using PCI wifi card) and my laptop I have lag spikes. This is a screenshot of the spikes http://i.imgur.com/cyoLq.png. Although it says its my hardware problems, I doubt its true, as it happens on both devices.
It is highly likely you have a interference issue on the wireless. It can even be your own machine interfering with each other. Games constantly send data which is one of the worst things you can try to do on a wireless. Wireless only allows 1 machine at a time to use it but it depends mostly on luck to determine which particular machine can use the service.