Several things could cause this.
1) you could have a underground leak in the main or branch lines.
2) A sprinkler head could have come loose and ism losing both water and pressure.
3) It could be a volume problem. Good pressure to start and then slowly loses pressure. **OR**
It could be as simple as a partially closed valve. I would check the pressure zone by zone to localize the source of the pressure loss/. Good luck. Tom

Thank you for your prompt attention speedball1. I will try this and let you know of the outcome. Have a great wkend!

Let me know what you find out. Your input will help others with the same problem. Good luck, Tom