Thank you for your answer, Jackie!
I've searched the threads and googled but didn't find much about this problem. Those that were about similar troubles were left unanswered or had the usual "download the latest version" and "clean cache" and so forth.
I still think it's pathetic that apple can not be clear with these things because I can't sit here and read every thread from people having problem with this and that and understand that it all has something in common with my problem. When searching for i.e. "iTunes will not play the entire song", as I've done several times, there should be a little something alerting me that it is a problem known to them and that it's under investigation.
It's not the first time I've had problems with itunes and every time it's the same; you have to go completely mad to recieve information and/or help.
"Apple had recommended uninstalling and then reinstalling" -When and where? I haven't seen that information anywhere.
Thank you, Amazon Jackie!