Loop Selection Question

With a midi item when changing BPM I cant get my loop setting to stay locked to the measure. For instance a midi item from bar 3 to bar 4. If I loop select bar 3 to bar 4 and then change the bpm the loop selection will stay the same length but its starting position will change i.e. no longer bar 3. This means as Ive got a loop going and I change the bpm to hear the looped portion at a different tempo, it all goes wayward. Is there a setting anywhere does anybody know?

Project settings: Timebase for items/envelopes/markers: Beats (position/length/rate)

Cheers, Perfect.