LTC 3813 2KW

Hi 

I use LTC3813 to do 2kw boost converter ,but i saw low side mosfet PWM as below picture,its frequency change,this behavior is normal or abnormal?

bulk voltage=75v ,and connect to 12V DC/DC load 20A

CH1: Low side MOSFET PWM

CH2: BULK voltage

CH4: Boost choke current

And, i found data sheet advice add resistor from Ioff to  Ith to fix this issue,but it can really fix this behavior?

Base on above description, can you suggest any ideal to fix this behavior?

BR,

Arthur