He has a problem... whether it is mental or caused by chemical imbalance I dont know. Its impossible for someone running for the US Presidency to constantly be making mistakes on the same subject over and over again.
If he does have such a problem the odds are that we will never find out. It will be hidden from the public. The same type of situation arose with the former Chief Justice of the US Supreme Court Judge Rhenquist.
FBI Documents were used as part of the story
snip
Long before Associate Justice William Rehnquist was nominated to be chief justice, he had a serious health issue that may have impaired his decisionmaking. For nine years, from 1972-81, the Capitol physician, Freeman H. Cary, had prescribed a powerful hypnotic medication, Placidyl, because of Rehnquist's chronic lower back pain.
This powerful controlled substance is prescribed for relief from insomnia. Its known side effects include "confused thinking, impaired memory," and even "delirium." Rehnquist started at 500 milligrams a day, but the dose soon tripled, to 1500 milligrams a day
http://www.law.com/jsp/article.jsp?id=1167818524831