I had the same issue with my Sako 90 Adventure in 30-06 (synthetic stock) while checking zero with Paul Hill of Corinium. Paul torqued the stock screws and problem solved, commenting that Sako have sadly had this issue since the 85 and the fact it's still present, he couldn't recommend. I've had a dozen or so outings with the rifle since with no further issue, but am watching very carefully and it's only a long standing affinity for Sako rifles that has me persist. Paul said exactly the same as you, that once experienced, he could never have confidence in the rifle again.