voltage dropping

I have tried the webench simulation with your condition, and for both 1A load and 0.83A load, the circuit can start up to regulate the output at 90V. If you meant the problem with real circuit which was built against the webench design, you seem having hit the peak current limit. Please double check your current sense signal at the Isense pin. If it is distorted by noise, an improved layout and filtering will be needed. If it is not the noise but the sense voltage is high, you need to check your sense resistor value and may need to reduce the resistance to avoid hitting the peak current limit.