Memory Card in Phone :/

Hello,

I recently got a new sandisk 4GB micro memory card for my samsung eternity phone. My phone reads the card for a little while and then when i go to play music it tells me i don't have a memory card in my phone. If i shut my phone off and turn it back on, my phone reads that there is a memory card but then later on in the day will say I don't have a memory card. Any thoughts??