The clock would seem to be a temporary loss of voltage to it, but could also be a radio fault.
P0128 is a start-up diagnostic code that indicates the ECT has not increased at the proper rate. It could be getting fooled by the changes that occur due to elevation change.
It sounds that it does not happen after a cold start when staying at elevation, or when driving after a cold start at the lower elevation?
The next time you head down the hill record the IAT and the ECT at start-up and at the moment the fault appears.
Sky NA 2007 Midnight Blue
Sky RL 2008 Carbon Flash - Naked