2002 Ford F-150 oil pessure problem

I have a ford f150 that the first time I start my truck in the morning it will read oil pressure but as I start driving it will show none and jump back and forth between having and not have pressure. But I can stop shortly after the initial start up, turn off the vechile for 2-3 minutes, start it again and not have any problems for the rest of the day, what might be the problem and how do I fix it