As a general rule: Vertical stringing is a pressure issue, and horizontal stringing is condition related. Eraticgroupings tend to be bending or scope mount issues. In BenchRest circles the thought process for vertical stringing is to either increase the powder by a click or two or decrease the charge depending on whether the conditions make you think that the pressure is high or low. for example if you did all your load development on sunny days that the temp hovered around 80 and this day the temp is a cool 60 then the assumption would be to bump the charge up a click or two etc.

I would further assume that since you stated that you haven't cleaned it in 1000 rounds that there is significant fouling. Significant fouling can cause increases in pressure, which might account for your stringing. That being said a Browning A bolt is far from a BR rifle, it is obviously a fine shooting piece though. I wouldn't worry too much either way if the group is stil under 3/4 MOA.