Underground lawn sprinkler problem - surging

My system has five zones. When I turned the system on today, the first zone worked okay for maybe 30-60 seconds, then all five heads started pulsating up and down, about 5-10 seconds per cycle and the "throw" of the water was shorter than normal. I walked back toward the manifold valve housing and could hear the surging all along the lines, which are about 10" below ground. No bubbling at the surface to indicate a leak below. The other four zones work okay.
I haven't made any recent repairs (replacing a cracked pipe or pop-up head) involving that zone, which has been okay for the season.
It seems to be an air-in-the-pipes problem. I don't know how the air got there, or why the problem just occurred. I guess I should remove one or two of the most distant heads to try to "flush" the air out. The most distant heads also happen to be at the lowest elevation. The lawn slopes down maybe 1.5 feet between the front of the house, which is where zone 1 is, and the 75 feet to the sidewalk.
Total pipe length from shutoff valve to closest head in Zone 1 is about 100 feet, and to the most distant one about 150 feet.
It's an old installation, at least 13 year old. And over the years, I have replaced many heads and sections of pipe (3/4" PVC).
Thanks for your suggestions.
R1
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Here it is less that two hours later. I forgot to turn the controller to its Off position, so the timed program started again, Zone 1 first. No surging this time.
In case it happens again, I'll swap zone wires as suggested. Nice tip.
Thanks,
R1
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