Vista - ReadyBoost, Vista SP1 computer freeze

I don't know if this is the correct place to poste this thread. I've been using Vista ultimate with a SD HC 8go card of which 4go are dedicated to readyboost. It worked fine except when putting the computer to sleep or hybernate. At wake up, it became impossible to open any application or even shutdown. Then Windows update installed SP1 and the matter worsened. Now if i leave the computer on its own, and the screen saver starts - computer even don't need to sleep - the same phanomeanon occurs, i.e it become impossible to shutdown the pc. The same occurs if i watch a DVD. One way though to unfreeze is to plug out / plug in the SD card, there seems to triggerr a reset sort of, and everything is back to normal. I wonder how could a avoid this ? Thanks in advance