Router providing less bandwidth

Since recent firmware update my AirPort Extreme is erratic in the speed it delivers to the home network. I have checked and my internet provider is delivering 60Mbps to the modem, but the network receives download speeds that can vary within a few minutes from 1.9Mbps to 17.2. That is a small fraction of what the network usually provides (very close to +- 60mbps. Unplugging and restoring router usually returns speed to normal, but that quickly degrades again.