Hi, 5 GHz doesn't penetrate wall/floors very well, so may be slower than 2.4 GHz.
Open Console in Utilities & see if there are any clues or repeating messages when this happens.
Open Network Utility in Applications>Utilities>Info tab, select Airport/Wifi & what is the Link Speed & are there any Errors/Collisions on the right?
Open Activity Monitor, Show:>All Processes, sort on CPU%, see if anything using too much CPU% when this happens, click on Memory tab, do you have many Pageouts?
you may also want to try changing the wireless channel that your access point is traveling on. if additional wireless networks have been added in your immediate vicinity, you may be trying to climb over one another on the same channel. i used to have to do this every once in a while at my home.
this is a change you only need to make on your wireless access point...not on your client devices.
I'm not sure how Apple (or any manufacturer, for that matter) determines which channel to try first when 'Auto' is selected. I usually select the channel manually...just a personality quirk, i guess.
i used to use a netgear router (2.4 band) and would experience interference on a pretty regular basis. i picked up an airport extreme on black friday because my local apple store had them on sale for about 150, and i've had no issues since. i set up a single network name for both the 2.4 and 5 bands, and interference has dropped to zero.
There are over ten times as many channels available in the 5 than in the 2.4, so the likelihood of interfering with another network is that much smaller. I originally balked at even the BF price, but the fact that i haven't had to worry about it since setting it up was worth the premium i paid.