An ampeg 8x10 in good shape might be 4 ohms, but I don't know since I don't use ampeg cabs. If you're not sure, then check it out and make sure - these amps just don't like less than 4 ohms. If this just started happening as opposed to "always done this" then the cab would be my first suspect - you could have shorted out a driver and actually be running at less than 4 ohms.
It's been a while since I tore apart a concert and I really can't remember exactly where the thermal cutout is located - but i should be making physical contact with the heat sink. But the next thing I would do is tear it apart and make sure there isn't an accumulation of dust, hair, dead animals or just plain crud all over it. These things need to be clean to do their job properly and any junk built up on it is just going to hold the heat in. If that doesn't fix the problem, then I would replace it - since they do wear out over time.
That big thick flattened V shape piece of metal on the back of the amp is the heat sink. If you can get the amp to cut out check this and see if it feels hot to the touch. In order for the heat sink to do its job, it needs to have free air flow behind it.
Other than that, the amp may have a problem anything from a shorted output transistor to a bad interstage transformer. Take it to a tech.
Installing a fan is just a coverup to what the real problem is... and if you don't get that real problem fixed it will come back and haunt you.
Good luck!!
