Bad ground, might need to A find a better place to ground the amp or run a 4 gauge wire from the engine block to the ground wire-make sure the engine to frame ground strap is in good shape.
If thats not it, could have some bad spark plug wires.
All else fails, a noise filter might be needed.
Does it do it playing CD and the radio or just the radio ?
Make sure the amp input level is not set too high.