Why would the Schmitt trigger relaxation oscillator a bit unstable?
Data sheet oscillation frequency is 5Hz, not 2Hz (K~=1). 

A bypass capacitor is not optional especially when you're drawing so much current through the LED (a long USB lead will act like a transmission line). Try 100nF in parallel with 100uF electrolytic, and go back to the original circuit with one gate. 

It's probably squegging all over the place without a bypass cap- but as you have no oscilloscope that would be hard to see.