wifi speed yoyos

I have a wifi network with an Airport exreme as the base station, and another extending the network. For several years the download speed has been in the 15-20 Mb range. Recently it fell to less than 1 Mb. Atest with the cable company shows the incoming signal is good (~60 Mb). After restarting the base station, the rate went up to ~30 Mb, but within an hour was back down to 3 Mb.

Is this kind of fluctuation normal? If not, what steps should I take to stabilize it, preferably at the 30 Mb level?

There are three other macines using the network, but there is no change in speed as they are turned on or off.