Photoresistor Night Switch
As it says on the linked page, "The magnitude of hysteresis is inversely proportional to the R5 value, being zero when R5 is open circuit."  So if you need more hysteresis to reduce the chattering of the output, you should decrease the value of R5.

You may be fighting against external feedback:  The light illuminates the sensor when on.  If there's enough of that, it's hard to get the circuit to work.  So you might benefit from making sure very little of the controlled light falls on the sensor.