Engine stumbling and dropping oil pressure..

I have set the timing by both ear and also vac pressure. The max pressure I can get is about 15 psi. I am not sure what carb it is other then I know it is a holley and I cannot find any numbers anywere that cross to anything. However, the Vac constantly bounces between about 14.25 and 15 psi and I cant seem to steady it out any more then that. I have a 318 on e-bay right now that I built for my 85 ramcharger, and if it does not sell I may just replace the oil pan and the pickup and swap it over into the dart for now since it is a completely new rebuild (I would rather sell it and get a 340 though). The odd thing I still cannot figure out is why it starts to stumble and drop oil pressure once it has been running for about 30-34 minutes....