1995 Toyota 4Runner 95 Runner-losing oil presure

My 4runner has 261000 miles overall. I had a crated engine put in 10 months ago. Last month the head gasket started leaking so I had to have a new gasket put in. The issue now -since I got it back is its losing oil pressure. I have normal oil pressure when starting-my idle is a little high so oil pressure is slightly high until normal temp kicks in and idle goes down. However, after 20 minutes or so of normal highway driving the oil pressure goes down and almost down to nothing at a stop. When I accelerate it goes back up but it doesn't appear to go up to normal pressure.
My oil pressure was fine(that I know of) before I had to replace the head gasket. Is it possible something was not put back together correctly? I had an friend do the head gasket work and ended up taking it to the service shop because it ran so bad after. The timing was off by 8 degrees and the "o ring was damaged which caused a fuel leak so I'm just wondering if replacing the head gasket could effect my oil pressure?