He is probably clearing a bounce and the added momentum will sometimes cause numbers to go down faster and earlier and steeper.
I think you're doing everything to try to stop these dives: feeding at +2, testing, etc. Maybe he needs a heartier/more/higher carb (not HC - just higher carb % but still LC) PS meal. It's all about experimenting at this point. Although, someone with more experience might advise differently.
But when kitties bounce, they bounce. I have a serial bouncer and your SS looks very similar to how Ming was in the beginning. You just need to keep trucking. I always asked why but often there is no why. It's just cats being cats.