PC Vocal Recording Stretches Longer than Backing Track

[slipnot]

I've been using online karaoke site for a few years now (recording with headset pink/green jacks in front of base unit). The recording quality's been really good..record voice then use mixer to sync the vocal track & backing track. Just lately if I get the sync right at the beginning of a song, the voice gradually falls behind the music track (i.e. stretches). I know it can't be 'drop out' or it would be shortening the recorded track. Does anyone know what's changed or if my PC has a fault. I've tried altering the sound settings but no difference. Thank you for any advice..