insane RAM usage

after an hour or two my games starts lagging (FPS wise) and if i look at my taskmanager it uses 11GB RAM (i only assigned it 1GB in the launcher and my JAVA has 4GB assigned. I only get this problem with 1.6.2 on 1.5.2 i rum at 1GB with shaders and all that. can anybody tell me a solution for this?