My avr-435 is about 10 months old.  When listening to movies and cable at a volume level of -20 to -17 dB everything is fine, when listening to music at a louder volume -10 to -8 dB for around 5 minutes my H/K just turns itself off.

Any advice?

Have you tried double checking all the speaker connections, so that no open wires are touching the other (including the ones on the back of the receiver), or that they are all in phase. Even if that's something unlikely I think it has to be checked. When the HK turns itself off, does it say anything on the display, like overload protection? You mention while movies and cable, so does it only happen in 5.1/7.1 surround and not stereo or does it happens no matter what configuration with those sources. Which speakers do you have, could one be 4 ohm like the 633t? Any recent changes to your system that could correlate when this started happening. Oh, another thing that I can think of is if you are using a tv or something connected to the plug outlet on the back of the receiver that draws more than 100 watts. Not sure if the 100 watt limit is for both the plug outlets on the back of the HK (one works while the machine is off and the other while it's on) These things are just off the top of my head, but an expert might be needed, you have a 2 year warranty I think.