There have been tons of posts on dropped frames, and I wanted to post a possible solution. I'm sure a few people have suggested this, but again, tons of threads, lots of solutions.
I've been having an issue with dropped frames out of the blue - a project I'd been working on for a week with no problem (and the same clips the whole time - I never added anything new) suddenly started dropping frames left and right. It kept starting at the same place, and I couldn't figure out what the problem was. I reconfigured a few of my clips, but it made no difference.
It was one of my Motion files. All I did was save it under a different name and then import it and replace the old file, and suddenly EVERYTHING works. No more dropped frames. Not sure what happened, but it's possible I made a change to that Motion file and it didn't reconnect properly?
So give it a shot if you're having an issue with dropped frames.