Paul, first...your 928 is an auto tranny, right? ok, then you have a kick down switch under the gas pedal. If you can get down there look to make sure the pedal is making contact with the switch when you push the pedal all the way to the floor. If no, there are adjustments on the pedal to make it do this. if yes, then test the wire connection to the kick down switch with a multimeter to make sure the switch is getting juice and working properly. I do not recall is this is naturally open or closed circuit, though you will figure this out when you test it.

This switch will push the car into a lower gear when you floor the pedal. The engine must be warm for this to happen. this should be noticeable when you are driving. If the switch multi meter test is ok, though it is not down shifting, then it is likely that you have an issue with the linkage either in the bowden cable or the WOT. That said, even if the kick down is working fine, there still may be an issue with the bowden cable. I would be surprised if your issues are not resolved with these adjustments.