Too low of a pH will cause you issues in coco...this is true, but, don't venture too far from that 5.8.
I'm curious as to why you're cutting back your ppm's when from the picture it's very clear you haven't even kissed the tips of the leaves with any sort of burn or over feeding. I'm not suggesting you up your feed, but I don't see where lowering it from what you've been doing is going to help. When you're trying to pin down a problem it's best to leave as many variables the same...small changes, one at a time.
Coco is one of the easiest mediums to reset. If you're worried about your coco being at the wrong pH, ppm level, salt build up or anything else, all you have to do is mix up the proper level of feed you want, pin down the pH you want, saturate the pot / medium with this new mix until you have adequate run off (more so than a typical feed / water situation) and you're all set to go. The medium will then reflect just what you put in and then the race to check your run off is a moot point.
I would still lean towards a Ca issue and if it were me, I'd find a way to add a little without adding additional inputs. Good luck
