Temp sender issue

[tdangle]

My sending unit went out and the gauge was not registering. I replaced the sending unit with a AC Delco replacement. So far as the engine is warming up it just keeps reading higher and higher until it pegs the gauge. I checked and the water passage at the top of the manifold is only about 180-185 ish. So I figured bad sending unit. Got another one and checked it prior to installing, comparing the resistance and water temp to a chart posted. Looked fine. Replaced it and it does the same thing, but as you let the engine run it drops to where it should be. I replaced the thermostat (180) with a new one thinking this was the problem. No change. Every thing checks out, but very unnerving seeing the gauge peg on warm up. Never had a problem with an air pocket before, wondering if this is the problem.