I don't know anything about the workings of the spoiler. But from what you described it sounds as though there must be some sort of "sensor" (that tells the computer what location your spoiler is in) that isn't working.
Since it doesn't know when to go back down on its own, or that it is up when you are going over 85mph. It sounds as though the only thing that it is doing right is go up when it's supposed to. But after that, the computer is in the dark and not getting the proper info, or signal.
I looked in my service repair manuals but couldn't find anything about it.
I hate to say it, but you may just have to take it back to the dealer.
Good Luck