iPod 3G in Dock Stops After Each Song

I restore old iPods and repurpose them with audiophile parts. I'm currently having an issue with a 3G iPod that I've retro fitted with a 128GB SD card.

When I connect this iPod to my O2 headphone amp (no power), it will play all day with no issues; however, when I connect it to an original Bose Sounddock (FireWire 12V powered) or an Altec Lansing im7 (also 12V power to the iPod, via the FireWire pin), and most recently a Kicker iK501, the iPod will freeze after I start playback.

The buttons will still make clicking sounds, and the currently playing song will complete, but the screen is frozen. When the song is over, I have to remove and reattach the iPod to get it to output audio again, or be responsive to my touch.

Is it impossible to have an iPod 3G charge and use it at the same time? I usually listen via headphone amp, but I'd like to be able to use a dock occasionally.