My old phone has become glitchy lately and often restarts by itself.

glitchy 是形容词，由名词 glitch（故障、小毛病）加上后缀 -y 构成，表示"容易出故障的"或"运行不稳定的"。在电子设备语境中常用来形容设备出现各种小毛病或运行不顺畅的状态。