How are you feeding the first round? Is the slide locked back or are you racking the slide with a closed chamber?
Upon reading post #12 again, since the nose dives are not caused during firing, and mostly the first or second (rarely) round. It tells me from experience its how the first round is stripped from the mag (compression and angle of the first found on the magazine),
it should not be any of the following, ejector or extractor as the spent case ejecting is not the causing the problem, nor is it the recoil spring.
Try this. When loading the mags to full capacity, give them a tap on the backside to make sure the rounds are fully lined up, and the top round is angled up. When loading the mag, do it with the slide locked back and instert with minimal force. Use the slide release to strip the first round. See if this solves your problem.
another method, is use a feeder mag, load the first round, then insert the fully loaded round securly. Giving you +1 capacity, then the fully loaded mag is stripped via the cycling action.
I am guessing you normally load the fully loaded mag into the gun, and racking the slide and the first round is then nosediving?