Is my SSD drive faulty when Blackmagic disk speed test shows decreasing write speed as low as 10 Mb/s?
mapasov pointed this link out for me to enable TRIM if you're using Lion http://digitaldj.net/2011/07/21/trim-enabler-for-lion/

It means that your system will handle TRIM as if it were a native Apple SSD, and that you shouldn't get the problem you're facing after the system has had time to manage it