Diagnosing Wireless Speed

A specific question, then a more general question:

Specific: I have a dual band router hooked up to a cable connection. I have several devices on wireless, 2 laptops, a phone, a desktop, and the television. The 5GHz band seems to be always reliably kicking out 15 Mbps or so. The 2.4Ghz band will usually kick out around the same speed, but almost randomly it seems to drop down to 1-2Mbps. It happens on all the devices using the 2.4 band. At the same time the 5 band will maintain the decent 15Mbps. I am attributing to channel conflict, but if I look at the other networks in the area (I live in city) they all seem to be using different channels. Any ideas what might be going on, or how to better diagnose.

General: Are there any decent home level (not commercial) tools to help diagnose wireless network issues. I'd like something that could help diagnose the source and cause of slowing. So it would measure at the router, via 5Ghz, via 2.4Ghz, and then tell me where the heck the problem is.

Thanks for any feedback.