usually when pressure builds up in the cooling system it is due to the coolant expanding, so after sitting overnight and cold to the touch the hoses should not be hard and the pressure should have released due to the coolant cooling and contracting to normal which it may or may not be doing, if it is then there is air/exhaust in the system coming from somewhere.
somehow air or exhaust is entering the cooling system, there is only 2 ways this can happen and either comes from a bad head gasket, to break the 2 ways down it is either exhaust or compressed air from the turbo, seeing boost is not what it used to be and i do not have a boost or exhaust leak visible on the exterior motor it has to be inside the block.
after doing a compression and a leak down test and coming to the conclusion compression is fine and it is iffy on the leak down there is either a bad head gasket, bent/stuck valve or bad piston rings.
since there is no white/black smoke it is hard to determine and the block has to be torn down.
i will test the radiator cap and thermostat but since i am not overheating and i know the temperature sender is working i have no other way to go but inside the block for examination.
hopefully it is just the gasket and the head itself is not cracked or warped, over a 100,000 miles and having the head milled usually ends up in disaster.
i can tell you this for a fact if it is the gasket then it just started and i caught it early enough to possibly have the head milled and not have to worry about issues in the bottom half of the motor due to improper compressions, etc.