RoJo
WKR
...seems like it gets worse the hotter the barrel gets.
Your SPS has a synthetic stock, right? I had a Rem 700 do that once, but it had a wood stock. It started after a bear hunt where I was hunting in the rain for 3 days. Apparently the stock warped, which caused a pressure point. The warmer the barrel got, the crazier it would shoot. A gunsmith re-floated the barrel, and he said he had to remove quite a bit of wood to do it. After that it was fine.