oil pressure gauge problem

So this is my first post . I'm having a problem with the oil pressure gauge on my 2000 v6 mustang. When i start the car the oil pressure gauge doesn't move. After 5-7 minutes of driving the gauge will go up to center and then back down, then after another minute or so it will go to the regular ok middle position. The weather here has been cold for a month or so however this problem just started a few days ago. I orignially thought that the cold had an effect on the wiring though I am a bit unsure. After doing a bit of research I hear that some people have had trouble with the oil pressure sensor in the cold. Any other ideas about what this problem might be? Thanks!

i figured it wasn't an engine problem, the car runs great even in these winter months. How easy is the oil pressure sender to replace? Can I do it fairly quickly, its damn cold around these parts and I don't have access to a garage.

Thanks for the help!