Strange performance of SSD

This is only a guess - because XP does not support TRIM, your sequential write is going over blocks that once had data on them and needs to erase them before writing them, this will slow down the write performance of the drive.

When you are performing random writes, the drive might be writing to blocks that are truly blank, resulting in faster performance because the drive does not have to perform an erase operation first.

There are utilities for Operating Systems that do not support TRIM that will go through the drive and erase data on blocks that are not storing information for the filesystem.

Wikipedia - TRIM

http://www.anandtech.com/show/2808/4

Those are pretty typical write speeds of the Intel X25-M.

According to Anandtech, a SSD controller that is optimized for random reads AND writes is expensive to make and implement. Since reads, especially random reads, are by far the operation the benefits the most from SSDs, Intel chose to favor that operation at the expense of writes.

From reading the glowing reviews about the X25, the write speeds has not hurt it. Keep in min these are still much faster than a mechanical harddrive's write speeds.