now that i can see the waveform i see why its getting confused, there are two places where the slope is appropriate for triggering the edge sensor...
Try different GND point - I'd go with pin #79 and then try something closer to the oscillator circuit if it's not clean enough. If that is still too noisy you can try bleeding the signal to ground a bit (with 100-1k pulldown resistor), this will lower the amplitude of course but it tends to eat up any parasite signals faster than the main one so it might just work.
