Another possibility could be that the new MSH is not allowing the GS to come back far enough (i.e., if the GS is not allowed to come back far enough, it will allow the hammer to fall, almost as though it is being depressed).
Try reinstalling your stock MSH. See how far back the GS sits. Then install your Wilson. I'd bet that the GS does not come back as far as it does with the stock MSH.
There are two "legs" at the top of the MSH. It may simply be that these need to be fitted in order for the GS to sit back far enough.