iPod error message after minutes of use.

I recently modded my 5th gen 80gig ipod classic to have a 128gb sd card as a hard drive, but I’ve been having problems. It will work fine but after using it for a couple minutes, the music breaks, the album artwork turns to white noise, and the tracks just skip. When I turn the ipod off and on again, I get the error message “Connect to your computer. Use Itunes to restore.” When I restore it, it fails, but when I unplug the Ipod, it works again for a couple of minutes, until the problem begins again. What’s causing this and how do I fix it?