Photos on my iPhone keep appearing and then disappearing

I have iCloud Photos enabled on all of my IOS and Macs. The iCloud library is fine, with the exception of my iPhone 6 Plus. I can have the Photos app open and I can watch photos appear and re-appear all the time. I have 42,000+ photos and I can watch it go down to zero and then back up to 42,000....although it takes about 90 minutes either way. When I look at iCloud Photos online, they are all there, as well as my iPad and iMac. Very frustrating.