the problem may be in the charge/discharge cycle. A capacitor's voltage changes 68% of the step voltage in the first tc (time constant, r * c = tc), so the voltage will go up very quickly and then will keep going up a lot slower, and when the opamp output goes low, it will go down very quickly so it is probably going below the threshold quickly.
try the circuit in post #6, but instead of putting a 1-10k to ground, use another 3.3M. This will give the discharge twice the tc as the charge cycle.