Single-handle Bath Faucet leak

It sounds like a Moen 1225 type cartridge. First is it "leaking" out of the tub spout or the shower head or is it coming out around the handle? If it is only the spout it could be a back pitch problem with the spout itself. It needs to be pitched towards the tub if only 2 or 3 degrees. If it's at the shower head only, it could be the diverter on the tub spout is not opening on loss of pressure. If it is in the valve body itself there a couple of things that need to be addressed. pull out the cartridge and check it all over -both metal and rubber o-rings & face seals for any small indication of abrasions or scratchs or slices or gouges. Hopefully there are none, but that would tell you where to look for a problem inside the barrel. Carefully stick your finger in the barrel as far back as possible and feel gently for any of the above mentioned items. Also use a very strong flashlight and look for any small pieces of rubber in the inlet and outlet ports. There shouldn't be anything in there but nice smooth brass and no foreign material. Then to dress the barrel slightly use a piece of 200-400 wet-or-dry sand paper rolled into a tube and lightly sand the inside of the barrel. Use a soft cloth to wipe out any leftover grit and putting dabs of Plumbers Grease on the end of your finger lubricate the inside of the barrel. Your new Moen cartridge should already be lubed from the factory, but it won't hurt it if you want to apply a little more. When you push the new cartridge in using the little white square head applicator turn it side to side about 45 deg from straight up and down until it is all the way in. If you want to use something other than Plumbers Grease just make sure it is acceptable for potable water systems and that it will not attack or dry out rubber. This should stop your leak. If not then the valve body was defective right from the factory or the original installer warped the valve body with way too much heat. In either case the valve (Complete) needs to be replaced. Good Luck