Walther Dominator 1250 Magazine Issue

Had an issue with my Walther Dominator 1250 recently so it went back to Umarex. Umarex charged me a whopping $80 to replace a ball and spring (also for a new bolt handle) since I voided the warranty when I opened the gun and modified the automatic safety. 

Since the gun has been back, I've been having trouble cycling through a full magazine. Seems like 5 of the 8 rounds cycle fine, and on the sixth shot the magazines spins to the left, which means no pellet goes into the barrel. This occurs with two different magazines and I never had this problem prior. It's frustrating because this had been the most reliable of the four pcps I own. Anyone have an idea what would cause this?

I do have a 30 day warranty from Umarex with the repair, so worse case scenario..I can send it back to them, which I really do not want to do.