My best guess, is that it's not the QB that goes from a 90+ QBR to half or worse, its the rest of game involved. Do your receivers have high bump avoid? Bump has become a key attribute for coverage. Play selection & matchups to def play calls are key, if you are spreading the calls out among a full active play call list, that can be a major problem in 4.5. Defense is now the name of the game in 4.5, you gotta have a good one or your toast.

What do you mean by full active play call list?

For offense, a full active play list would be 40 plays selected. Then it comes down to the "spread" or number of times used among the 40 plays based on your offense settings & rules.