Almost sounds like Syncro Rev match... it is possible that you have a leak in your idle air control valve. But there are several things I am sure that could cause a rising idle. It "could" be a downshift but I've never owned an automatic that downshifted while braking (it usually waits for you to reapply the throttle so it knows what gear to pick). My Z isn't an auto though so I am talking out my rear on that one.
Your best bet is to try and document it via video phone and bring it to the dealership. If you go there unequipped they will almost certainly tell you they couldn't replicate the problem.
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