RQL_NOT_LESS_OR_EQUAL & KMODE_EXCEPTION_NOT_HANDLED

I have a problem: every now and then my computer freezes or starts lagging heavily. Sometimes I have to wait a while for it to unfreeze, and other times it stays stuck, so I have to restart it. After that, it won't boot, and I have to restart it many times until it starts correctly—except it takes longer every time. I usually get the errors IRQL_NOT_LESS_OR_EQUAL and sometimes KMODE_EXCEPTION_NOT_HANDLED. What could it be?