Do you have the latest drivers? What is your OS? Has this ever worked before? What version of AE are you using down to the decimal point? Answering your question as stated is as easy as answering "Why is watermelon?"
It's a Mac Pro 2012 OS: 10.9.3
32gb Ram 1066 MHz DD3
NVIDIA GeForce GTX 680 2048 MB graphics
CUDA driver up to date. I had to close a load of tabs in Chrome which seems to have fixed the problem
Chrome and Firefox are real GPU and memory hogs. Make sure your AE is up to date.
yes, it's the latest one 126.96.36.199
I think it's chrome that causes hassles. Especialy as i had loads of tabs open (Youtube, soundcloud etc)