Moli
With other radar users on the forum not having this problem and some of them asking for transponders NOT to be set to Stby when changing codes I can only assume that it is the way that the D&D radar is set up
My guess is that the problem is not one of scrolling through the codes so much as the way D&D radar date is proccessed when it comes from the radar heads, my guess is that you are getting the data "raw" and without the two sweep delay.
The other issue is pilot training, your tyro pilot is told to change the code, he starts to do this, suddenly remembers what his instructor said and having already got an emergency code selected dithers a bit and then selects Stby by that time the damage is done and your (hair triggerd) radar set picks this up. I have seen this happen with students and if the guy had just selected the correct code without delay no ones alarm would have tripped.