I have had the 128G for about a year now. Write speeds are very slow however.
It takes forever to install a game in steam or any other windows application for that matter. Read speeds are fine (i.e windows 7 ultimate loads in less than 20 seconds from boot)
AHCI is enabled in my motherboard (asus) bios.
SSD is connected to a sata 6 port on the motherboard.
Indexing is disabled on the drive.
The SSD has been like this since I bought it (i.e it has not got gradually worse)
try secure erasing the SSD. That should fix any performance issue related to the flash. If you see the same speeds after secure erase then I would test in another machine and if the results are similar submit an RMA request to sandisk support.