Mid-to-high 30s is not his ceiling, but it has been his typical performance in Midwestern open primaries. Trying to draw a distinction between Trump's performance in Wisconsin and Illinois isn't particularly useful, because Trump got 36% in WI and 38% in IL. If the dates of the two primaries were flipped, Trump would've won Wisconsin while the opposition was still divided, and he would've lost Illinois to Cruz. Again, if Trump wins Indiana, it's through new supporters having been attracted either by distaste for the Cruz/Kasich deal or through his mid-Atlantic victories.
Trump had a terrible week because he was losing Wisconsin. He did not lose Wisconsin because he had a terrible week. His performance in Wisconsin polling did not shift at all during that week.
Trying to group Minnesota in with Wisconsin is even less useful, because Minnesota was a closed caucus and Trump performed way worse. The difference is incredibly marginal.
The difference is Trump has all the political and moral momentum right now. There are always going to be fence sitters that jump the winner's way.
Wisconsin was 1) a fluke, and 2) back when more states were voting. Trump also underperformed in the Midwest because the nomination process was still wide open. Now that the process is almost over, most people are going to want to go with the apparent winner and bring this to a close.