Ford Mustang: GT 2000 - oil pressure doesnt register, oil pressure gauge, weather engine

Question
Mustang 2000 GT, 4.6L, 777250mi. On cold start, oil pressure gauge sits on zero for 2 to 5 minutes, then jumps to middle, falls, jumps to middle and stays. Oil pressure tests fine at start. Remove sender wire and ground it, and gauge goes to 3/4 scale, never to full scale. My mechanic, who did all this, thinks it's some electronic module that controls under-dash gauges and other electronics. When this started a  couple of weeks ago, the following also began: cold weather, engine temp never rises above 1/4 of scale (was always at mid scale), and gas mileage has dropped from 19 to about 14. Is any of this likely to be connected, or is it the module for the oil gauge, new thermostat, etc (car fully tuned, plugs and all, at 70000). Thanks.

Answer
I think that it might have something to do with the cold weather having an effect on your wiring.  I also believe that the temp is probably your T-stat.  I would start by changing your T-stat and see if that fixes the temp. gauge.  If it doesn't it is probably the wiring harness.  Hope this helps.
Taylor