Mine strings for the first 5 rounds or so (2-3 MOA higher than POA) but then it stabilizes and groups well. Since this happens again after its been sitting a while such as after a target change, I believe it to be due to barrel temperature.
What type of stock are you using? Wood may have shrunk during the time in storage giving more wiggle room to the heel of the receiver. This could also cause POA shift and stringing.