Sandisk 8Gb Extreamly slow speeds after format

I really need some help. My 8Gb pen drive is giving a lotta problems.

It was working fine and used to give speeds upto 20-30mb/s and even more. But then i tried to install Mac OS X through it and formatted it in by MacDrive8 in Mac file system. But when i tried to formate it back to FAT it showed dat only 6GB was available even after format. So i used GParted Live to format it. 

Now i hv full 8Gb available again but the speed is about 2mb/s.  Can sumone please advise where did i screwup?  n how can i rectify the problem?

Thnx in advance.