Another possibility is that you could be pumping the pan dry. If you are running a stock pan with HV pump, it can happen and will look just like you described. I know it does not help, but I had this issue when going to a solid cam in a SBC a long time ago and restrictors did not help. I ended up putting the cam in a new engine and selling the old long block. The new engine is still running with no oil pressure problem and the old engine ran two seasons in a circle track car. I think that my problem was the HVHP pump with solid cam and stock pan. I came up with this since the new engine used standard pump and had no issues. At the time it really sucked because I could not figure it out.