this child won't sleep at night

Alright so I know this is common and probably just a wait it out situation, but thought I'd try to get some advice 
My 6wo flops back and forth with good nights (wakes up every 3 hours, eats and falls back asleep) and bad nights where she won't sleep unless held. 
She falls asleep while I feed her, and then wakes up and cries when I try to put her down. Her diapers are clean and she is tired so I don't think she is confused with night and day. She will sleep just fine if we are holding her but the second you put her down she gets so fussy and eventually starts crying. 
Any ideas or advice? It drives me crazy that o can't pinpoint what she wants. Also drives me crazy that what works for her one. It doesn't work the next night.