Challenging math problems are solved using a great deal of working memory, or "short-term" memory. Studies show that those who perform regularly with a higher degree of working memory are more susceptible to pressure because they rely on a working memory that has been crowded out by anxiety.
The solution to this is to either reduce reliance on working memory (higher reliance on automation or memorization) or become oblivious to pressure. Pressure seeps in through self-doubt and pessimism. Practice and familiarity can reduce the amount of self-doubt and reduce pressure, thus increasing thinking capacity in a high stress environment.