I was lucky enough to find a brand new amp online for around £100. An audio place might be able to fix your current amp. The problem (if it's the problem you've got) stems from the solder on the amp cracking.
Before playing around with the amp whip off both door cards and swap the front speakers over. I have the common crackling front speakers and played around with the soldered joints on the amp to no avail. When I swapped the speakers over the problem shifted to the other side so in my case at least it is the speaker at fault. Very easy to do and will at least give you an idea of where the problem lies before you start replacing things.