make sure the speakers you are using are 8 ohm...also Sony receivers have always been known to be limited on power output compared to their ratings...it is also highly likely that a frayed wire somewhere is shorting out when you get x (45) level of power running though it (the wires may not be touching, but once enough power is being run it creates an arch and shorts out causing the amp to go into protect).

and as always list the model numbers you are working with.