foF2 has been bouncing around in the 5-7 MHz range, which can cause this effect. When it goes to just above the frequency you're listening to, the signal booms in (maximum signal strength is typically when the frequency of operation is just below the MUF). Then it drops a little, and the signal quickly fades away.
You can tell this is the case by examining the signal on a waterfall, and observing selective fading of the sidebands.