The 7600 has different input and output circuitry, which gives higher performance but is more dependent on cabling.
Don't really think I can do anything for you unless you want to totally document your setup, describing all cables and connections. And, even then, it could be a faulty cable. Sounds like you need to have a good technician check out your setup.
You could certainly send the unit in, but my guess is that they will find nothing. However, I have been known to be wrong, so this is an option.