I'm on the final stages of a frame off build on a 77. I've put a 5.0 in it from a 99 Mountaineer and it was built with some minor mods like flat top pistons, cam and some head work. It was built by a reputable builder that builds racing engines. It has a Melling high volume pump and ARP rod that drives it. When I finally fired the motor up about a year and a half after it was built it had good oil pressure at first then dropped to about 7 lbs at idle and wouldn't come up with rpms. The lifters were noisy also. Pulled the front of the motor off to replace front seal and found two of the small pressed in cam bearing plugs out and were found in the oil pan. After threading the holes and putting threaded plugs in, the oil pressure came up to about 65lbs at idle and the lifter noise stopped. After running for less than an hour the pressure fell back down like before and the lifter noise returned. This has me baffled. Any ideas??? Thanks for your help in advance.