I forget where I read it but there were a few stories of amps which went into some kind of ultrasonic oscillation when the thermal protection circuitry was "almost" triggered, resulting in some tweeter-frying excitement.
I can't find a link or anything though...
I guess the other question is whether the amp should be going into thermal shutdown under the conditions you are operating it under. Does it seem to be getting pretty warm before it cuts out ?
As others have said it is extremely unlikely that you are getting a run of bad tweeters. More than extremely, in fact
