These drops that you are talking about are called air pockets.
I've experienced them a lot, Denver (DEN) is quite famous for them.
When I flew on Lufthansa's FRA-DEN flight, when we were beginning our decent, the plane kept dropping maybe 200 to 300 feet, then goes up 200 to 300 feet. They weren't comfortable, and many passengers on the flight got a little worried.
I had never experienced such bad turbulence on a jet before like this time in Denver, although Denver is always turbulent due to the constant changing pressures, and un-even air currents created by the mountains.
However, the bumpiest experience I ever had was over New York. I was 6 years old, so I am not sure what the aircrat was. I think it was a Saab 340. I flew from New York LaGuardia (LGA) to Rochester, and there were huge thunderstorm in the air. The whole flight was very uncomfortable, I remember the flight attendant falling. The plane dropped to 700 feet several times, and went back up.
It was the first and ONLY time I have ever felt sick on a plane, I nearly vomitted, luckily I didn't though.
Since then, I've been a little paranoid to fly on turboprops.
However, I recently flew on an ATR 72 from MIA to NAS, and NAS-MIA on an ATR 42, and they both were pleasant experiences.
"Generosity is giving more than you can, and pride is taking less than you need." - Khalil Gibran