Shopgeezer, what you describe sounds like the way I first tried to use telescoping gauges. Since then I have learned the correct way (or at least, I think it is the correct way): Put the telescoping gauge in at an angle, letting the fingers spread to touch the walls. Turn the handle to lock - which doesn't lock as much as hold with friction. Tilt the gauge straight - this will push the fingers in to the minimum measurement, which should occur directly across the bore. Tilt a bit more and pull it out. Measure across the fingers gently. Not sure how much sense that makes to describe it verbally. Quick and easy to understand if you see it, harder to describe it.
Of course, it may well be that you are doing it correctly (or at least as I describe above), and your gauges are still not holding the measurement, in which case disregard all of the above!