Why did you switch to a smaller diameter pipe halfway down the run? What is your static water pressure at the Point of Connection for the system? Generally, but not always, when you are on Municipal water you should have more than enough pressure for a sprinkler system. This is why it is very important to do your homework prior to installation, so you can figure out pipe sizes, the correct sprinkler heads to use, and the amount of heads per zone. A booster pump is probably only going to solve your problems temporarily. Until we know what your water pressure and GPM are though, its hard to say.
EDIT: On a side note, how many sprinkler heads do you have on a zone? Also, generally you want to seperate heads with different flow rates into different zones - IE. seperate large turf rotors from the smallers 570's.
Tony Posey
Ridge Run Landscapes