I just dismantled my sunroof a few weeks ago to cure a little leak I had at the base of one of the holes where the screw goes through to the retainer on the otherside.
By dropping, I take it you mean when you release the clasp and flip it up to "prop" the sunroof on it, hence when you hit a bump, the clasp flips back/or drops and the sunroof drops with it right?
I believe 100% your problem has to do with the springs on the clasp which retract and extend the hinge mech to the roof of your car.
In order to take your sunroof off your car, you need to release the clasp. With the clasp hanging down, from the back you should see this chrome plated hinge with hooks on each end that fit into slots in the plastic clasp on the roof of your car with a spring spooled around the chrome hinge. That spring is there to provide some tension for when you lock the sunroof either when you close/lock it, or when you open your roof and "prop" it. Sounds like either your spring is worn out, OR, you have NO spring at all, OR something on your clasp is either cracked or broken off.
If you need to change your spring, or the whole clasp, REMEMBER, first you must loosen the allen key bolt on the side of each phillips screw/bolt. There's one for each and they go through the plastic into the bolt. IF you don't do this first, you may shatter your sunroof. Once you've done this, you can unscrew and dissassemble your sunroof lickity-split.
Upon tightening on re-assembly, be very very gentle. You only need to tighten enough to make contact between the retainer plate on the outside of the glass. Add some sylicon around the holes to prevent leaking. The allen keys are what stop the screws/bolts from loosening. These you can tighten a little more.