Large mp4 video in ie 7

we are delivering a rather large mp4 video through flash, progressive download 40mb (I know a bit to big, but flash does it’s jobpreatty good).
It runs nicely on every machine in our range in firefox, safari and chrome, on ie however, it drops a lot of frames the first time we play it, ones cached it runs nice.

Any ideas why this is happening in ie?

My guess is that IE doesn’t do simultaneous reading and writing to the memory all that well.