Just papered a dining room in an old stone house. Inside walls hung fine, Outside walls have some bubbles after paper dried. Why does this happen, and what can I do to prevent it?

Bubbles are usually one of two things, air or paste. When you push it with your finger against the bubble, paste will leave an indent but air doesn't. If they're paste then it might be that the outside walls are a bit damp so the paste isn't drying out properly. They might dry out over time but otherwise to be honest you're going to have to sort out the cause of the damp. If they're air bubbles then cut a crosshair shape in the bubble using a sharp stanley knife and ruler. Lift the flaps and apply some paste then stick it back down and you should be ok. You can use a seam roller over it if its proving stubborn.