ASIO4ALL version 2.1

I don't switch between ASIO & MME - ASIO is on all the time. The issue I have is only apparent on songs with tempo changes - if there is a bar that slows down before the real drums come in on the following bar, or if there is a change in the style AND tempo. It seems able to cope ok with style only changes, but not tempo too. As I've said, this issue doesn't exist when playing the song directly through BIAB, only when it's been rendered using the WAV button on BIAB. Rendering appears to occur properly, could it be something to do with the merge - in with midi and audio tracks?