iTunes won't play songs I purchased from iTunes correctly. Interrupts playing all the time. Help please?

I purchased several albums on iTunes and downloaded the songs to my iTunes library on my Mac. Ever since I've been having trouble playing them. First iTunes wouldn't play the songs at all, then only selectively and now it only plays them with interruptions after every 2 seconds or so. I have authorized the computer and removed the SC folder, as suggested by apple support, but nothing helped. Do you know what could be the problem and how to solve it?