Slow pressure increase

Each of my six zones works at approximately 75% pressure for the first 5-8 minutes, then eventually ramps up to the usual area coverage. Never did this before. I read an earlier post about low pressure in one zone and the recommended fix was either a new solenoid or diaphragm. I'm wondering if that could be the issue for me as well.
I have a Watermaster 57223 auto valve model. 3/4". Due to placement, I'd hate to have to replace the entire sprinkler valve.

City water or your well?....Upper

City water. System is at least 5 years old. I have been using it problem free for 3 years.

So if you took 2 or 3 garden hoses and turned them on,the would not fluctuate greatly? Second all zones do this equally?Thirdly when a zone comes on and is lacking,go and manuallly turn the solin. lefty lucy and see if this helps...Upper

The hoses would not fluctuate greatly...pressure is solid at my house.
All zones do this equally. There are 6 zones total.
I haven't fiddled with the solenoids yet. It seems odd that all six would need a tweaking.
I tried turning lefty lucy and saw no noticeable benefit.

Thanks for taking the time to help.

Seems to me that if you're having the problem in each of the 6 zones that the problem is between your water source and the valve manifold. Very doubtful that the problem is in all the individual zone valves. Do you by chance have a master valve in the system? I would track your plumbing back to your water source tie in point. What kind of backflow device do you have? Is there a pressure regulator in the system? These are some of the things you need to check. Especially since it is a city water system, it should be easier to track down.