yeah, you have to keep in mind the sheer THICKNESS of those lines. The stock oil pump will not be able to handle that kind of flow and maintain pressure.
Jesus, I've seen thinner lines on a forklift mast assembly, or reach truck fork control lines!

(filled with, you guessed it, oil) Why on EARTH are you using such thick lines? You will need a higher volume oil pump for starters... see if you can get thinner lines, i can almost guarantee that will solve your problem!
Hook up an oil PSI gauge (installs in about 30 min), they cost like, $5 on eBay.