Apparently gravity feed is not producing sufficient flow; this is because of (a) slow air intake causing back-pressure (b)insufficient head (c) line restrictions not detected. The venting problem if that were it could be at either end--air has to replace the water moved from the tank, and must be free to miove out of the line at the other end.
You could test the head proposition by hooking up a lighter five-gallon thing on a ladder just to see what it does to your net flow.