I used a Shearer 3.5 in a G17 and had the same problem. I installed a NY1 trigger spring and it seemed to fix the problem. I've been shooting Glocks for over 20 years and had some misgivings about using aftermarket parts in a carry gun, so I put everything back to its factory original 5.5. Aside from a broken spring once in a while, I've never had any reliabliltiy problems with a Glock with OEM parts. The only exception are Wolff magazine springs, which have been better than original Glock springs.