Yes, the CYD has a small 4-pin port and comes with matching connector with 4 wires, one wire is ground. Strange thing - if I disconnect the ground wire from the CYD, the display shows a constant / stable 125.5 hz.
This is how I'm converting 5v to 3.3 v:
I'm using a 200 pf cap (not 22 pf). The scope ground, signal generator ground, this converter circuit ground, and CYD ground are all connected together. But when I remove the CYD ground connection the display is stable 125.5 hz.
The CYD is powered from USB cable connected to a nearby computer. So there must be some interaction with the USB ground and the signal ground. I will try to power the CYD from a battery next.