I tried your application. I get pretty much consistent framerates for all cases which is from 90 ~ 150 FPS. With no backbuffer it sometimes jumps up to 900 ~ 1000 FPS but only for a brief moment. The only problem I've encountered(other than the low framerates) is that the DIB backbuffer does not display anything at all.

As for the game I get around consistent 1000 ~ 1100 FPS in windowed mode, for fullscreen I get 1800 ~ 2100 FPS though it sometimes jumps briefly to less (like 1400 FPS).

I am not sure why this is happening. It's quite bizzare. I think I've heard of something similar before but cannot remember. If I do remember I will post it. As for your timer, it seems kinda irratic. I'd suggest that you get the framerate once per second, instead of lower times you might be using.

As for my hardware, I have an AMD Athlon64 3000+ and an ATI Radeon X600 Pro (Catalyst 9.3) with 512 MiB RAM.