Jdownloader is a great program and the only problem i ever had with it was not down to the program but the version of java my PC had. I updated to the latest version of Java and the program runs without problems. Try updating Java and see if that helps.

I have a premium rapidshare account as the free account is pretty useless. Rapidshare allocates free download slots and if they are all full then either your download speed will be really slow or it just won't download until slots become available