How do I tell if my truck stereo amplifier is broken?

I have an Infinity Reference 5350a and both the power and protect LED's pulse in irregular pattern and the speakers occasionally go fuzzzzz THUMP! It will play music if I unhook it and it sits for awhile. I took it to Car Toys where I paid a $50 diagnosis fee and they said it is likely broken. But they also stated the speaker was shorted out when I checked and it wasn't. It is only a year and a half old, It never overheated, I don't abuse it. How can it be broken so fast? Is that common? Is there any way to fix it that would be cheaper than a replacement? thanks in advance.
 
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