DS is 7, considered G&T but not doing anything special at school (though the school is very good and pushing all the children so they are not bored) and sometimes he can quickly answer sums, apply maths, tell you sensible things about the time and day and everything else, but at other times has a total blank, talks gibberish, words in the wrong order, could not tell you his birthday without you breaking it down (what is the month? what is the day? what is the year? and he says ohhhh riiiight my birthday is xxx as though the question had been double dutch) and tells you he can't do really basic sums even if he is counting on his fingers. I think it is genuine, not playacting, most of the time. Sometimes my brain works better than on other days, for sure, but this seems a bit extreme. Has anyone else experienced this?