You've introduced a new bug :) The first black line is no longer significant. I don't have to drag the red ball past it before releasing the mouse button...

By the way, it's possible for the inner part of the wheel (the coloured bit) to get separated from the bike. Move back and forwards enough times and you'll see it happen. Probably you're making all parts react to key presses in the same way? However the key might be released between one actor's act() method and the next actor's. Also, the wheels don't turn at all when the bike is rolling along, which is a bit disconcerting :)

Check you "about greenfoot" dialog - the greenfoot.properties file is in the same location as the greenfoot-debuglog.txt file.

Regarding the pauses: I am seeing network traffic while the pauses are occurring. It probably means you have images/sound files in the wrong location. Using the latest version of Greenfoot may help.