Do you know more about the vocab test your Ds took? Was it like an IQ screening test? I don't know much about tests, but sometimes IQ scores can be unreliable under ages 4-5, especially if you have a kid who is very verbal at a young age. That said, it seems an awfully big change to go from 99.9% to 75%. It sounds more like maybe it was just a bad test day for your DS. Does it seem wrong to you even if you don't consider the earlier scores?