Amp problem

Hi peeps, I have a problem with my amp and it's annoying the hell out of me. My sub was working fine then cut out, I turned it of and back on again and it works again for a few days, today it cut out again. I turned it off and on as I did last time, there is a slight 'thud' from the sub then it goes into 'protect' mode. I've checked the fuses, wiring and connections but still doing it. The power light comes on when ignition is first turned for about 3 seconds then the power light goes off and the red protect light comes on. I thought it might be the earth but it has a good spark when I connect it so that's good I think. Any ideas?

Where is the amp earthed? Ive only had one amp go into protect and it was because of bad earth. If its happened 'out the blue' id be going round and checking all connections are tight, and run a multimeter past power/ignition/earth for continuity.

Worst case - have a sniff at the vents on the amp- any burnt smell would point to it being fubared

Thanks Skinz, I checked everything, earth is good and no splits on any wires. Tried a different amp from a friend and works fine so must be the amp is knackered.