Slow render with 2.66 and gtx580

My system has one GTX580 now- when rendering the BMW benchmarks scene the GPU takes 1:05 when according to the benchmarks online it should take ~40 seconds? Here is what my GPU usage looks like:

It bounces between ~55% and 99% cyclically- any ideas as to what is wrong? I have the newest driver, and tried all sorts of tile settings… I am about to pick up my new computer with 3 GTX580s and I want to make sure I understand how to get the most speed out of it.

Have you changed the default render tile size? Tile size guidance can be found here:

definetly check out the tile sizes… disabling aero will also help a little as well . but the main difference is that blender has evolved since the benchmark first came out, cycles has more features which inherently makes it slower…to put it in perspective, when i made the benchmark doc, my card was 46 seconds rendertime… now, its 56 seconds. I am thinking of making another benchmark + thread for the latest blender… my plan is to make a script that tests gpu / cpu & submits it correctly to an online database.

As I said I “tried all sorts of tile settings” from the default to rendering it as 1 tile… what do you mean by “disabling aero”?

Is it normal for the GPU usage to oscillate like that?