- Does this happen from a "cold start"?
- Is it possible the amp is overheating due to ventilation?
- Are all the speakers rated a 8ohms or some 4?

If not, the surrounds in movies take a lot less power than the fronts and center channel, so doing the 3xM200's should fix the problem. In fact even a 7100 should drive those speakers with no problem. Also you could drop a M200 on the center channel and try that first. A lot of the output hits that channel.