Knowledge is noor. Suppose you have read some thing about the law of diminishing returns and then you have forgotten about its exact form. Even in this case you have the noor of your earlier knowledge. Even if you have to revise it completely the earlier noor is not lost. We can take this as a blessing because we are supposed to acquire knowledge from cradle to grave. Our pious predecessors preserved our Deen for us by their hard work. And what is that? Revision and doing round with others. You must have seen the Huffaz - they sit down for half an hour or so for revision and recitation. You have to keep revising. That is how you retain - there is no other short root. This is true in secular sciences too. I know a person who is an expert on theoretical technique. The published review article on that topic is there in the library. That article has been read so many times (by him and others) that the pages bear visible signs of excessive browsing. Cycling and swimming are some thing that you do not forget in a life time. Programming you forget in a weak. Revise, revise, revise. Recite them all everyday. That also is true. And at such moments we should remember the Qur'anists - they think that we can not remember what we ate two weeks ago. This might be true but then there are those things that we revise everyday, like Surah Fatiha. And then the things like you have mentioned. In both of these cases our memory is working wonderfully. As I said - we got to understand the way it is right. (1) Revision of what of the Holy Qur'an we have memorized and (2) may be we should try to write in a diary those past events that come back to our mind out of no where and they are as vivid as they were on the original occasion.