fl2k output voltage level changes every few seconds

Hi. When running the command line fl2k_fm -c 5000 /dev/zero to generate a continuous wave, the output voltage of the fl2k dongle regularly switches between two different amplitude levels (peak to peak voltage) every 2 or 3 seconds, by about %30 to %70 depending on the load. There's no visible gradual transition, it just switches instantly. The output peak-to-peak voltage is about 350 milivolts with no load. I've calculated the output impedance to be about 750 ohm at that frequency for the larger output level. I'm using the HDMI version of the dongle with the wire soldered after a capacitor (or resistor maybe) which was already there, to the red VGA pixel output of the chip. Could the issue be because some signal conditioning circuitry is missing that is present in the actual VGA version of the dongle, or that the pin is actually pulled to ground in this dongle as it is unused, and the chip doesn't like it and for some reason it then decides to regularly switch voltages? Any other ideas?