Great news finding the problem. When you tighten the chain when bike is up on the center stand you should have about 1 1/2" of play measured as follows;

Find the center point between sprockets and with a rag push the chain down.
Measure the distance from the ground to the bottom to the chain.
Push the chain up and measure the distance from the ground and the bottom of the chain.
You should get about 1 1/2" of play.
The chain tightens up when you take it off the center stand and sit on it. Have a friend sit on it and see how tight the chain is.

 

Hear ya , did just that had the two kids on it, also I have loaded the spring up more
as she was too soft, or I'm downing too many summer refreshments......not sure which LOL

But needed stiffer rear as there's a couple of bridges on my commute with expansion
joints that give you that "jockeying" see-saw ride if the rears too soft and I hate slowing down

BTW

Thx Sprock