Intellibox 2 sudden speed increase

[2foot6]

Hi,I have an Intellibox 2,sometimes after setting the speed of a loco,say at  53,after a few seconds the speed will go to 126,I then turn back to 53 and again after a few seconds it will go back to 126.This happens on a number of locos with different decoders...it does not happen all the time but it is an annoying problem.My question, is there a reset code or fix for this problem............cheers Peter