Not sure why the tube would slide inside the tyre unless the tyre itself were slipping round the rim and taking the tube with it. The usual solution is to dust the tube with talc before installation, which reduces adhesion between the tyre and tube.
Also make sure that the tyre bead seats properly next to the valve. Sometimes you need to push the valve in slightly to make sure the bead doesn't catch it.
Not sure why the tube would slide inside the tyre unless the tyre itself were slipping round the rim and taking the tube with it. The usual solution is to dust the tube with talc before installation, which reduces adhesion between the tyre and tube.
Also make sure that the tyre bead seats properly next to the valve. Sometimes you need to push the valve in slightly to make sure the bead doesn't catch it.