Maybe you need a break, try to do something different before getting to your study.

I find that problems that I know how to do 100% of the time... i sometimes get wrong because if I've done them before because I'm trying to remember the exact way I did it previously and/or the answer I chose the last time...

You should definitely take a few days off and do something relaxing.

Cinji18,
I really have to keep my mind fresh during these problems or the same thing happens to me.

I agree with Money - sometimes you get questions wrong when you are trying to remember how you solved them in the past as opposed to simply solving them. But sometimes you identifed a core concept that is evading you a second time around. This can be helped if you identify the concepts invovled in a problem and note how you recognize these concepts and the steps or tricks that are related to that concept.

Thanks, everyone. I will take a break, then try to do what has been suggested.