I have the same problem and would like to know if the original poster found a solution? For me it seems that the speed bug never makes change over from IAS (showing 315 kts) to mach speed when in VNAV mode, eventually resulting in an overspeed condition if left in VNAV. The only work-around I've found so far is to change vertical mode to LVL CH, manually press the change over button, and leave it there until CRZ altitude is reached where I can switch back to VNAV. If I switch back to VNAV mode after doing this, the speed bug reverts to 315 IAS. The problem only started after I made a fresh install with SP1. I've tried a handful of different combinations, both starting cold & dark at parking and with engines running on the runway, and with two different routes, each one manually typed into the FMC. In each case all the relevant pages on the FMC has been filled out and there is no indication from the FMC or instruments of anything being wrong along the way (except for the missing change over and resulting overspeed).Anyone has any idea as to what I should try do next to resolve this issue?
