Hi there, since no one has pointed this out I just wanted to say you may be simply experiencing regression to the mean. You did so well the first time that you most likely had some element of luck (i.e. they just happened to ask you the things you knew). I don't think you are getting "dumber" by any means. The thing is that when you get these sorts of stratospheric scores there is often luck involved and thus it's very hard to duplicate or beat the score later. Does that make sense? For example, in high school I made a 1480 on the SAT, 800 V and 680 M. My mother wanted me to re-take so I could break 700 on math, but I refused because my verbal score would have almost certainly been impossible to duplicate because of regression to the mean; I simply had an element of luck the first time.
As far as the theory that there is significant difference between the various NBME forms (usually the theory is that form 1 is the easiest and form 3 the hardest), I have not found this to be the case. I've been taking one a week to guage my performance, and took form 1, then 2 then 3. My scores followed a more predictable trajectory, going from 230 to 240 to 253. Since I DIDN'T start off with an extremely high score like you did, it was much easier to go up. I will take form 4 this weekend but I won't be dissapointed if it goes down a bit--I'll just chalk it up to regression to the mean since 253 is a very high score (You still managed to beat me by one point w/ your initial score though, congrats)
Hope this helps. Obviously no one "knows" for sure what the predictive value difference between forms is but in my experience they all felt about the same. Hope this helps or reassures at least one person here!
