The cube I had been rendering did not calculate for camera pitch/yaw, so if it was a 3d game you would only be able to look in the +X direction. I added some math to fix that, but then I got another problem.
I wasn't using floating point numbers, and I was using a homemade arctan table which, sadly, may have been less than accurate. The cube jumped around the screen and was recognisable only most of the time.
I was using propeller spin language, and used fixed-point arithmatic. I think its time I used c++. We shall see how it goes!