Bucket rendering display inconsistency between vray frame buffer and max frame buffer

[stevesideas]

Im having to use the max frame buffer to be able to save out my render elements (not sure If I need to but I cannot find a way to save them otherwise). Anyway, thats not the problem.

The strange thing is, if I choose the vray frame buffer, the rendering is fairly consistent and gets to half way at about 22 seconds (of a 1920x1080) render. The render takes 48 seconds.

If I choose the max frame buffer, half the render is complete within 10 seconds then suddenly the buckets chug and the render starts taking longer, the final time being 48 seconds.

What is going on ?

[stevesideas]

Ive just bought up the windows task manager and looked at the cpu usage during a single frame...very wierd indeed. It starts off at 44% usage and as the render continues the cpu usage drops to 10%......next frame starts, the cpu usage is above 50% then gradually drops down as the buckets continue....