Reverb audio render problem

Hello All.
Using Davinci 14b Studio
I am trying to render a timeline with 2 stereo channels and 1 Video.
Whenever I render a movie with an audio VST (everyone I've tried so far, reverbs, autotune etc) the rendered audio is 'choppy' and 'broken'.
Live playback in Davinci is fine however.
Any clue why it would do this?
I've tried rendering to different containers but all do the same.
Has anyone experienced this?
Many thanks.