If the fuses were the problem then you would be replacing them all the time. Fuses are only good once - and are only in place to protect the circuitry from damaging overcurrent situations.

More likey, the problem is a thermal protection resulting from over-driving the amplifier. What impedance are the speakers you are running? Does the amplifier support that impedance? Typically, car audio runs 4-8 ohm impedance. Many amps have a switch to place the unit in the proper impedance mode.

Check your wiring. Do you have it wired correctly?

Check and see what size fuses it uses. Your watt power means nothing without good fuses. My suggestion would be to go buy 40a fuses and replace the ones you already have.