High End Computer and lags and crashes after 10mins

Got a new pc yesterday as my laptop with a integrated graphics card gets 4-5fps due to 1.6.1 and there is no solution yet. This computer runs the game smooth about 120fps with max settings (Fancy - Far - Smooth lighting on maximum) BUT...Every 10-15mins of game play my fps drops to 0 until i restart my minecraft. after i restart it becomes better if i logout of my pc then log in it becomes 120fps again when It slows right down its spams my developers console with this error below over and over until i restart:

Client> 2013-07-05 15:03:51 [CLIENT] [SEVERE] @ Post render
Client> 2013-07-05 15:03:51 [CLIENT] [SEVERE] 1285: Out of memory
Client> 2013-07-05 15:03:51 [CLIENT] [SEVERE] ########## GL ERROR ##########

I have tried allocating more ram to my minecraft and lowering my settings so i'm sitting on the 250-280 fps range. same thing happens.
Please help as I only got this pc to moderate my server, because my last one is to laggy from the 1.6 update.

Plz reply dont shut this and not give me a solutiong and leave me hanging :O