It sounds like a frame rate issue or your computer is not fast enough to play back in real time. Does the raw footage itself play back normally? You said you’ve checked your comp settings already so maybe double-check that the imported footage is being interpretted correctly by right-clicking on it in the project window and going to Interpret Footage>Main.
To check if your computer is lagging, do a RAM preview with the Info panel visible and see what the playback frame rate is and also if it says “realtime” or “not realtime”.
Erik Waluska
EAWmedia