Having just had a NEW higher end scope bit the dust quickly and suddenly, thought I would post this..my experience was almost identical to what was described..shot a good tight group..4th shot dead center with group but 2 1/2 inches low..then back up to group..then back down..

Tried factory ammo that had consistently been shooting around an 1" group--now 3" plus group..
Only the 3rd time in my life I've had a higher end scope go..so not bad odds but still frustrating..

http://www.accurateshooter.com/optics/diagnose-scope-problems/