Dough breaks before second rise is finished

Hi folks,

Even though I peaked in here a few times over the last couple of active sourdough years, this is my first post. So far I was able to always find an answer to my problems, but this one is giving me a head ace since a while and digging in my personal recipe and procedure might give me a better answer / solution from you guys. 

So I am baking on a commercial micro bakery level since a couple of years (around 100 loafs a day for 2 days a week). Since my set up is pretty basic (no mixers, no proofers, only commercial convection oven), my breads are either high hydration tartine style miches, dense grains loaded rye breads or a simple overnight proof (room temp.) no knead sourdough pan bread.

The later is the one giving me sleepless nights since a while. The thing is: although I am measuring my factors (temp., time, etc.) every time to get consistency, I have very different outcomes from one week to another. Between beautifully risen loafs ready to get baked up to half risen ones starting to rip/crack and fall of at the end of the second rise (not giving much of an oven spring). 

So I guess I just gonna lay down my recipe and process and let you guys shoot your ideas.

4900g organic white flour (all purpose)
3400g water (90°F)
100g salt
600g liquid started (peaked after 5h rise)

1.) At 5pm I mix everything together (no autolyse)
2.) I set room temp. at 73°F
3.) I give S&F every 30min for the next 2-3h
4.) Around 3am the bulk is at around double the size (everything looks fine so far). I separate the dough and let it bench rest for around 15min before putting it in the loaf pans, covered with humid clothes to not let them dry out. I try to not handle the dough to much - no real final shaping, just forming a tight ball with my bench knife.
5.) Then, while baking my miches for the next 4h, I let the pan loafs rise at around 80-85°F. Temp. is hard to control in my small bakery with 3 ovens running at 500°F, but I figured out how to keep the temp stable. 
6.) At around 8am the miches are done and I am ready to put the pan loafs. But here is the thing: some batches rise very well - doubled in size (slightly over the rim of the pan with no cracks) and spring nicely in the oven after. Other batches, like the last two weeks, rise around 50% (2-3h in the rise) and start cracking all over. The never get over the rim and shot almost no oven spring after.

So any ideas out there?

Here are some pictures:

 

Obviously those are the good ones

(full risen)

(not even fully risen, but already breaking) - note: the glossy surface is due to sprayed water just before baking

(les risen, flat top, broken structure, more dense crumb)