TOP258GN questions (flyback, 11W)

Gentlemen,

i've got a few questions about TOP258GN.
It is built into a 15W flyback unit. The circuit has 2 outputs: 24V/0,25A and 5V/1A.
Feedback is done by TL431 and optocoupler. More or less it like a is a school-circuit.

I cannot stabilize the behaviour of the circuit. In case I switch the power on/off, sometimes the circuit stops for a while. But not always!
As I decrease the input voltage, this kind of stop comes more often.
I think, it is due to the current limit, but I'm not sure.
Current limit is not deceased, M pin is connected to ground via 4k7 resistor.
I discovered, that the dI/dt value is larger in my circuit, than is mentioned in the datasheet. Obviously it hangs up on the input voltage, but at minimum input it is double than specified in datasheet. As the input voltage increases, the dI/dt goes up above 1A/usec value. I do not measure the switch current, instead this result comes from a calculation based upon the primary inductance of the transformer (datasheet parameter of transformer).
Also, the output noise contains the input DC bus's ripple.
Next question: the ON time of the switch is not a fixed value. It changes always (load is a fix resistor), like a random value, between 0,8 and 1,2 usec. Is it OK or this is an oscillation in the control loop? It looks on the scope like a jitter.
Sometimes I see on the scope that 2 pulse comes together and then a little longer pause. I don't know the reason. Some audible noise comes too.
I tried to reduce the gain in the feedback loop, got somewhat better, but not correct. I also tried some damping capacitor without proper result.
The whole feedback comes from before the PI filter.

I hope someone has a good tip. Sorry for the long post.