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LM3278

I have a circuit that has a LM3278 dual regulator. The input is +18V and the output is +5V. There are supply issues, so I changed the FETS from FDS6982AS to Vishay SI4936BDY and the startup waveform looks odd. The regulator will not always start up. I am attaching the waveform of the input and output voltages.  What is the interpretation of the attached waveform.?   

Thank you very much for your help and support.

Jeff Cohen, Hubbell Power Systems, Boonton, NJ