Is each speaker connected to the same + an - terminal on the amp?
If so, then each speaker is in series with the amp. So you're amp is running 100W to each speaker at 3 ohms.
If the speakers are both connected to the same + and - terminal, then you are in parallel. So you're amp is running 1.5 ohms and providing 200W.
If this is the case, then this is why your amp cut-out. It is unable to provide that much power for long periods of time. Also putting a 1.5 ohms load on an amp no designed to go that low is harmful to the amp.
