1998 Audi A4 low oil pressure

Thanks for your quick response. The motor was rebuilt due to high milage but also because when I had purchased it, the vehicle was hard to start. The intake cam had to be replaced(badly damaged). It also looked like the old owner had a timing belt break and then unloaded it. It was knocking when I finally pulled the motor. I replaced the cam bearings and completely rebuilt the head as weel as the motor. Cam clearance could be an issue. The thing that is confusing me is the idea that the oil pressure is good when it is cold but when you reach operating temperature it becomes low. Would bearing clearance be effected by temperature? Like I first thought, maybe an issue with the relief valve? The main bearings have been done twice, once with the new motor and once again yesterday, I did this incase the oil pump may have caused damage. The other thing is that when I first started the car after the engine replacement it seemed to take an abnormally long time for the lifters to pump up. I am going to guess at maybe 4-5 minutes. This is why I thought that it may have been the bearings. Please let me know if you need any more information, I am DEEPLY in debt with this car and it's problems, but I cannot sell it like it is, besides I did like the car before these issues. Thanks again for your response and help.