Odd minecraft lag

[wkingsnorth]

May not be the right place for this, but:
I have some weird lag on minecraft - I have a custom high end gaming pc but minecraft lags every few seconds, even on minimum settings - any reason why?

System specs:
i5 2500k 3.3ghz
AMD Radeon 6970
8gb 1600mhz ram
Custom Liquid cooling system
silverstone 850w strider plus PSU
coolermaster HAF X

 

[VeNT]

minecraft forums has some details, seems like the more ram you add to java the slower it gets, best works when forced to use 256mb

 

[wkingsnorth]

how do you force it?