Wow! You need to get your FC to STAY between 5 and 10 ppm. Until, then you need to shock it up to 20ppm 2 to 3 times a day. That's with CYA=80ppm. If it's 100ppm, then you should be between 8 and (I think) 15 FC and shock to 25 (won't hurt).

You are fighting something and that's why it's dropping. You need to maintain shock level chlorine until it's dead. Meantime, filter on 24/7, brush and vacuum every single day until it maintains a steady FC--and then continue the treatment for 2 more days.