Which player are you using to play the songs? Is it LW's media player or iTunes? ie: Try playing the songs with either iTunes or another player such as Audion (click on link)

If you're using LW's media player, then if LW is too busy the player suffers. So this might be the cause. LW devotes most of its resources to searching, downlding, uplding, etc. which is its fundamental purpose. Most people in OSX don't use LW's media player for playback.

I've heard of this problem before although I've never experienced it. It's either a player issue or dud song files. T3 Spammers; mp3 Bit-Rates Versus File Sizes Google Overpeer to get an idea of what I mean.

BTW if you're not a presently paid up user of LW Pro then LW Pro support will ignore your email.