Chanalyzer + Wi-Spy DBx
Generally regarded as the "cable tester" of Wi-Fi, a spectrum analyzer should be the first serious step that you take when you begin working on a Wi-Fi issue.
Chanalyzer (the software) and Wi-Spy DBx (the hardware) work together to bring you portable spectrum analysis. With a spectrum analyzer, you can look at the raw radio frequency activity around you - not just where the wireless networks like you can with a normal Wi-Fi adapter.
Eye P.A. + AirPcap Nx
Ok, so you've looked at the spectrum, and didn't find any non-Wi-Fi interference? Good. Step 2 in most Wi-Fi troubleshooting processes is to check out the actual 802.11 conversations with a packet analyzer.
Eye P.A. can take a packet capture with an AirPcap Nx (a product by Riverbed), or you can bring your own packet capture. It parses the data to visualize which networks and clients talked a lot, and which ones talked a little.
You can use that to find out if a device hogged all of the available airtime, and you can quickly drill down into conversations to see 802.11 conversations.