tempo rushes

My playback as I cycle through a tune, starts off correctly and then begins increasing with each new cycle. It starts at 168 and on the 2nd cycle jumps to 180. 3rd cycle is 192, as so on. Does anyone know about this or how to fix this? I certainly seems like a bug.

I use this in my improv class and after a chorus or sometimes two the tempo begins to markedly accelerate to tempos unplayable by the students.