If the static pressure is 42 psi, then you have an uphill battle, and anything that takes away pressure will hurt system performance. So you might certainly consider upsizing the plumbing, starting with the long runs of pipe, both indoors and outside. That said, a flow of 6 to 8 gpm is hardly a killer for 3/4-inch plumbing. If it happens that the neighbors have 1-inch water meters with 1-inch incoming, and you have a smaller meter on 3/4-inch incoming, then you aren't ever going to equal the flow and pressure they have in their sprinkler systems.
You also want to be certain that none of the drain valves in the zones are leaking. You might try capping off all the head connections on one zone and see what results
One thing that a do-it-yourselfer can try on one zone as a last resort, is to switch heads to the best low-pressure performer of all, the Maxipaw impact. They are rarely a pro choice except on 'dirty water' supplies, as pumped from a pond, but they are out there, and they will throw further at low pressure than anything else, period, which gives you the flexibility to tweak their diffusing screws for extra in-between coverage. You would change the stock blue nozzles in favor of the black ones, and adjust the arm springs for low pressure.
This post has been edited 1 times, last edit by "Wet_Boots" (Aug 8th 2014, 8:25am)