one thing I've done...I think Vanman may have suggested this originally...draw the object at 90, set your ucs to the slope, and use the 3DAlign command. Even with flanged pipe, I use the router line to get the slope in and then move the. Also, I think sometimes if you set the UCS XY plane in the same plane as the slope, it may revolve.

Again, and old AutoCAD error, perhaps they've fixed it in some scenarios, but not in all.