Welcome to the forum.

Congratulations on the choice of 3" 686. Once it gets straightened out, it's a great gun.

What's happening is not normal. I regularly run sessions of 150rds in my 686's, with no variation to trigger pull. Have you removed the cylinder and made sure everything is clean and moves freely?

Bear in mind that your new 686 has an excellent warranty. A short phone call to customer service and they will email you a pre-paid FedEx shipping label, and FedEx will come to your door to pick it up. If it's clean and full functioning to start with, then I'd send it back for them to look at.
My 3" 686 had to go back twice before they got it right, but now it's a favorite. Their turn around was about three weeks.

Some feel that extended use of lite weight rounds like your using are not optimal for the guns design, and believe 158gr is the better weight for plinking. Has to do with flame cutting.

Good luck to you and your wife.