One of the zones in my backyard has low pressure. This zone has pop-ups, and all the rest have impact heads. The pressure in the popup zone is too low to fully push up the heads so the lawn never really gets watered.

Here's the strange part: If I unscrew one of the popup heads and let it run for a while (10-20 seconds) then the flow increases. You can actually hear it increase in the house (the sprinklers run off city water.) After that, I shut off the water, screw the head back on, turn the sprinklers back on and they work great. So the pressure is there, I guess the valve is not opening up all the way...? A friend suggested air bubbles might be the culprit.

Is there a test I can do to check and see if the solenoid valve needs cleaning/replacing without digging it up? Is there something else it could be?