I got the folks that installed the pressure reducer back to my house. After they try to tell me again that my sprinkler has nothing to do with it, they try to convince me that my original pressure reducer NEVER worked and then it broke worse (started making the horrible sound that caused me to call them). Somehow I avoided strangling him and demonstrated my most recent discovery of how the shower loses all pressure if a toilet is flushed. He considered the idea that the pressure reducer was defective. He replaced it and everything works.
Somebody is getting a nasty letter from me. Thank you all for supporting me in my time of need.