My money's on your second conjecture -- increased network activity during busy hours, ESPECIALLY if you live close enough to neighbors such that their WiFi activity can interfere with yours.
If you have access to your router's setup, try configuring it to use a different WiFi channel to see if this helps the situation. In the 2.4 GHz band, channels 1, 6, and 11 do not overlap each other and thus give you the best chance at reducing congestion, so try each in succession, although depending on what channels your neighbors are using other channels may work best. Also, reducing router bandwidth from 40 to 20 kHz may also reduce interference.
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