Unstable in convert DC - DC chopper
The main problem is you have C1 in the wrong place. It should be put just after the rectifier to smooth out the DC voltage, but you have it across the Zener diode which will cause a large current spike each time the transistor is turned on. 

A secondary issue is that your scope is not measuring the output voltage. To see what the output waveform looks like you can use two channels in differential mode (eg. C+D on channel C and INVERT on channel D, with channel D connected to the lower side of R4).

Note that the output is 'floating' ie. not referenced to either ground or supply positive. It will be difficult to regulate the output voltage in this configuration. You might consider using a PNP transistor or MOSFET to switch the high side instead. then the output will be referenced to ground.