100w /17.5v = 5.7a of possible current flow.
17.5v /5.7a =3 ohms.. What is the resistance of your load?
I think it needs to be 3 ohms or less..
The 12v wallwart power supply I used had caps on the output.
So, it's 12v output was stable, even when the PV voltage level dropped.
You could try adding 100 uF between pin 8 and ground,
and also add a 10 uF between pin 3 and ground. (across the zener diode).
That might stabilize the IC power and the reference.
It's been years since I blew up my circuit.
So, I hope my suggestions help..