I have a 94 S500 with 173K miles on it. I changed the oil tubes last year. This summer I have noticed on longer trips at highway speeds over 75 MPH, with the AC on, and the engine temp over 95C, my oil pressure gauge goes to 0 and the red light comes on. When this happens, I take my foot off the gas, the needle jumps up to around 3. The engine purrs like a kitten, so I assume it is a bad oil pressure sending unit, or bad wiring when it gets hot. Oil pressure readings are normal for other everyday driving. Can anyone offer any different thoughts, and a procedure to change the sending unit. I know where it is. I assume I need to get under the car to do it.

Thanks