Engine stumbling and dropping oil pressure..

Not 100% sure, and I don't know your engine size. I do my timing by ear and Vac pressure. I have both a 273 and 318 that idle about 750 RPM (carb adjustment), however my vacume at idle is about 21 PSI. Before adjusting your distributor Vac screw, I would adjust your timing to get Max Vac. Advance or retard your timing while watching your vac guage. When at max PSI, back off about 1 psi of Vac. and give it a spin around the block.