@popey It looks like it's not capturing the horizontal blanking/sync pulses properly. Perhaps the blanking pulse voltage is below the trigger threshold of the capture double but above that of a television. If that's a Sinclair Spectrum, it's pretty old and the voltages could be off due to components shifting out of spec with time.
@popey It would be rather easy to see what's going on if you had an oscilloscope at hand. You might also want to examine the voltage(s) in the power supply of your Sinclair with a multimeter. If a voltage is somewhat low, that could explain why the dongle isn't handling it. I reckon those dongles aren't used with old hardware like that very often. They probably tested it with some camcorders and game consoles and called it a day.