Now do this:

You say you have installed a new inner tube and inflated it to about 40 psi. Now that you have done that, examine the tire all the way around on both sides. Most tires have a line running around the circumference near both beads. If both of those lines appear to be equal distance from the rim, you're good-to-go.

I agree with the folks who suggested you have pinched the tube somewhere under the tire bead. When that happens, the internal air pressure tries to blow the tire bead off of the rim. Sometimes it's successful right away, sometimes it takes a while. Checking the distance of the line in the tire from the rim gives you a warning if the air pressure is trying to blow the tire bead upward.