vacillate
v[I, Ipr]
~ (between sth and sth) (fml usu derog 文, 通常作贬义) keep changing one's mind; move backwards and forwards between two emotions (思想)动摇不定; 犹豫; (在两种情绪之间)变化不定
She vacillated between hope and fear.她时而抱有希望, 时而心存恐惧. Cf 参看 oscillate.
It's quite simple. You put a very hot, wet cloth on your shoulder for one minute. Then put a packet of frozen peas on the shoulder for one minute. And so on and so on.