I took it to Glock. They replaced the front & rear sights, firing pin, recoil spring, trigger bar, locking block pin, firing pin spring, slide cover plate, and tossed in a new Glock magazine with a +1 extension. He said he replaced some of the parts that probably were not needed.
Took it to the range this weekend to test out. Works great with the original 2 mags that came with the gun and with the new one Glock tossed in. I went to shoot with my backup mags (a G23 13 round and a G22 15 round). I had FTF to feed on the 3rd - 4th rounds every time. So I went back to the short (original) mags and all was perfect. I shot the G23 & G22 mags in my G23 gun several times - no failures at all. So back to the G27 with the G23 & G22 mags - FTF everytime - starting around the 3rd round. Prior to my original issue - I was able to shoot these mags just fine.
Any ideas or is another trip to Glock in order?