I am trying to use the tempo blocks to match up beats with a song. This song has a very well defined beat at 140 bpm, but it seems that setting the tempo to this is far too slow. After much testing, it seems to work closest at around 149.5, but still always gets off beat.

Is this just me? Is there any way to reestablish the beat, like reset it from the timer or something to put it back on beat?
Thanks, Bitasy.

The tempo will just define how long a beat is when you use the play drum, play note or rest blocks. If you do other things between blocks then they could add delay. Can you share your project to see what is happening?

Here: http://scratch.mit.edu/projects/19963804/

Yes I think this is a bug as well. You're not doing anything else blockwise and even if you try turbo mode to eliminate any lag from the forever loop it is still out.