About Karaoke Tracks

I have a problem when I try to record over Karaoke tracks. I let the BGM track play out and simultaneously record my voice on another track( to be later mixed). I use Audacity recording software. However, I find that there is a time lag when I try to play it. I've tried to move and align my voice track with the BGM track but somehow its not perfect - the worst part is...even if I manage to align the beginning of it perfectly...I find that I have messed up the timing badly...like several times it sounds like I am trying to hastily catch up with the beat/timing. It's frustrating. Its not like recording with music is entirely new to me. I have about 1.5 years experience recording for devotional albums...movie song tracks (they just call them tracks - they are actually substitute singing to help in futher instrument mixing etc. Voice by a popular singer is finally added to the mix after they remove my voice) when I was in Chennai. So I don't understand why this is hapenning. Makes me have real respect for all you guys out there successfully posting songs recorded over Karaoke. Any suggestions welcome.