buck regulator MOSFET turning off for a short duratiopn
You need to look at the data sheet you linked to.  Page 11 explains burst mode.

The effect you are seeing is caused by the chip using burst mode.  When you are only drawing a low current from the chip, it will shut off the drive signal in order to save power.  At higher loads the drive signal runs continuously.

The sample circuit from the datasheet has burst mode enabled (BURST_EN grounded.) 



In response to comment:
See page 11 of the datasheet, and see what it says about Vc.  Take a look at what Vc is doing.  Place a larger load on the output, and see that Vc rises.

Your load would seem to still qualify as "low load."  Given that the example circuit provides 75W of output, and yours (3.3V at .3A) gives only 1Watt, this seems reasonable.