Tā de bìngqíng fǎnfǎn fùfù, ràng jiārén shífēn dānxīn.
His condition kept going back and forth, making his family very worried.
Tips
grammar
反反复复 is an AABB reduplication of 反复 (fǎnfù, 'repeatedly'). This reduplication intensifies the meaning, emphasizing extreme repetition. Other AABB examples: 高高兴兴 (happily), 马马虎虎 (so-so).