The way I see it, you are improving. There could be several factors as to why your score dropped. You could be doing really well in some areas, while there is at least one area you did not master. Hence, when you do not get as many question from the area you are not good at, your score tends to get higher. My advice to you is to figure out the area you are not good at, read that part of the guide with more care and do more problems.
Also, you mentioned that you did a CAT day before and scored 750. Well, let me tell you something. Take GMAT as a marathon. When you run marathon, you do not run it twice in two days
Your brain is probably tired and thus it is harder to concentrate.
My advice is that you need to get some rest from taking tests and cover your weak areas. You are doing well and I think you will make it. Good luck!
Set your aims high, carpe diem