I have been running DC consoles inside 3 different style cabs with different monitor manufacturers, and never had this problem of inconsisitant brightness from on DC game to another.
It may be that you have it set to low to begin with and its less noticable on some games than others. *shrug* Or maybe it takes awhile for your monitor to warm up? Is it inconsistant from one day to the next with the same game inside? May just me a monitor problem/issue.