Disconnecting in android

I bought a 16gb micro sdhc sandisk from walmart for my t-mobile comet and everything went fine till the next day it just randomly disconnected and i had to reboot my phone for it to show up. Reformated the card same thing. Tried a diffrent card (8gb) and i dont have that problem.  My phone supports up to 32gb and thee disconnecting happens at random sometimes hours apart sometimes minutes apart.  Please help 

You can use h2testw to test the card for errors/bad sectors. Although rare, it may be defective.

I ran H2testw and no errors.   From what im noticing it seems to be after the memory card is used and it goes back to rest / nonuse it sometimes disconnects. The problem seems to jump around to diffrent things.  Now its the sd-ext part of the card wont mount correctly