Callback when a frame renders

Hello again. Is there a function or a way in Unity3D to get a callback whenever a frame is rendered? Right now I have some code in FixedUpdate() that drives a special effect but now that our frame rate has dropped the effect doesn't really work anymore since too many updates happen in between renders. Basically I want to force a change every frame and not have it based on any sort of physics that's usually driven by FixedUpdate(). Thank you -Mo

Update runs every frame, but there's also OnPreRender / OnPostRender.