I just built a 400 small block and it runs fine but the oil pressure has me worried. when its cold and i start it up it reads 50. but when it gets up to temp its almost 0 at idle about 25 when im driving any ideas?  

The rule of thumb I've heard and read with highvolume oil pumps is 10 pounds per 1000rpm.Do you know what your bearing clearances are at?Were you recommended to use a certian viscosity of oil?  25 pounds cruising at 2500rpm? Tell us more   WIEDER  

Oil pressure will always read higher when oil is cold, But running almost zero at idle when engine is warm does pose a problem. 25 to 30psi. at normal driving is about normal. A few easy things to check first are make sure valve cover breathers are clean and not clogged up or if you are running a pcv valve make sure it is working properly and not clogged up, and make sure your oil pressure line going to your guage is clean and not gummed up. It sound like to me there is something not breathing right which will caus the low pressure at idle. If all of those things are good then the next place to look is inside the motor. hope this helps a little  

Regardless of your volume, oil pressure will stay the same as stock as long as the pump has the same number of vains and the bearing journals are all near stock specs.  Repair manuals will show you that a small block chevy has an oil pressure min. spec. of 6pst at idle and 20 psi at 2500 rpm at operating temp.  If you're running a stock style pump with stock-type internal parts, I'd say you're doing ok....as long as it's not making any noise.  

Either the sending unit for the gauge is bad, or you've got some severe problems in the bottom end. If changing the sending unit doesn't do anything, sounds to me like you've got a clearance problem in one of the bearings.

Are you hearing any knocking noise when the motor's running?