I'd suggest you have a look at what's going on using something like omnipeek. This will give you information on what's actually happening at the packet level. If you're on an open network (or have provided a PSK for WPA2-Personal secured networks) you'll also be able to see higher level information which may indicate the source of the problem.
These things can be really time consuming to resolve.. I spent a long time troubleshooting an intermittent application problem with iPads where they'd freeze up for 30-40 seconds. Never found a smoking gun, but picked up half a dozen things to tweak on the WLAN while I was at it... and the problem just stopped being reported.