Might need some more info, but here's what I can suggest right now:
1) Check all your cart slot connections to make sure that nothing is bridging, and that all wires are connected to the correct pins, etc.
2) Are you using the original AV cables on a TV or have you soldered to it? If you have soldered to the AV port, make sure that your connections are not loose, bridging, and are touching the correct contacts.
Yeah, what's your exact set up? Are you using a TV, or are you soldering to an LCD? Are you still using the power brick, or have you solder in any voltage regulators? Also make sure to test with another game if you have one. Sometimes our games can fry or bork on us. Make sure the pins on your game are clean.