Hi Greg
I'm no sailing scientist but what jumped out at me in your last post was
"The main was centered with the mainsheet eased a lot"
There are two major issues here, first you could do serious damage to the top of your mast as the stresses from the head of the spi aren't being supported by some leech tension on the main. The second is that you have significantly closed the slot between the wind leaving the leech of the spi running straight into the back of your main.
I would suggest that it is better to keep alot more mainsheet tension on and drop the traveller 2-3 inches in moderate winds and up to 8-10 inches in light winds and not at all in anything over 15mph.
You never know doing the above might stop the forces on the main from giving you feeling that the boat wants to skew into wind?