AHCI is definitely a better setting so its good that you made that change.
If the SSD is benchmarking correctly and the SMART is not showing any problems it seems like the SSD is working normally. You could try a secure erase (that should reset the drive to out of the box performance and reinstall to see if there is any improvement.
The problems you are experiencing sound to me like they could be video card related. I would suggest updating your video drivers and see if there are any changes.
