Anybody heard of that a Triac could "jam" so it's always open. But only does this sometimes? So if You don't give the gate any V, the triac acts like it has V anyhow, but only sometimes!?
Yes, when coming spikes from AC line, the triac can firing for a half period randomly. This thing depend from dU/dT ratio. The RC or MOV sometimes help (parallel with MT1 and MT2).
The motor is a brush-type? If yes, it can generate continous spikes... If it is an older, good working circuit and now malfunction, possible failure of MOC or triac also.
Anyway, i use always two antiparalel SCRs for inductive loads without problems.