CMDLCache :: Out of Memory

I've recently had this error pop up whenever I load a map up in Hammer. This is really irritating since it's the map I'm working on for the Artpass competition, and after spending quite a while on it having to lose all of it is really irritating.

System Specs:

Windows 7 64-bit
Intel Core 2 Quad 2.50GHz
6GB DDR3 RAM
NVidia Geforce 220 GT

Can anyone help?