The Bugaboo is not the ideal stroller for bumping up and down stairs, The reason being it's core design. Most stroller handles have two or more mounting points to the frame but most strollers have baby facing forward and facing the big bad world and not Mummy. The Bugaboo is very clever, the handle can be locked ether way and the seat face either way but the handle has only one point where it attaches to the frame and that point has to rotate and lock. To give you an idea of the forces involved a crow bar the length of the Bugaboo handle can pull six inch nails out of hard wood!
Unfortunately Bugaboo have not used the best material solutions in this critical handle lock. They use a plastic disk to move aluminum locking pins and they fail if you bump down to many stairs. This happened to our Bugaboo in New Zealand. I repaired ours and now I help others by making or sourcing the better quality parts to repair them. I use aluminum for the disk and stainless steel for the pins.
If you are lucky another to have a Bugaboo less then two years old and still have proof of purchase documents Bugaboo will replace the frame for you. If it's older you can buy a new frame for around $400 but this will also increase you resale value by around $200 when you sell it. Lastly you can repair it for around $100 a side depending on parts required, Click on my name and you'll find many posts on how to repair both the Handle lock and the Handle extension lever or wing-nut.