Lanfear isn't playing with her power level nor did the Finns tinker with it, Moriding place a TP Shield on her nor the Dark One punish her - she is hiding it to suit her own purposes, which she has done before. Thus there isn't an outside agent fiddling with it - she herself is making others perceive her as weaker. An example of this would be Aes Sedai's reactions to Mesaana - Mesaana hides her strength in the Power so they can't figure out which Forsaken she is nor how strong she is in relation to them, but neither she nor they doubt she is stronger. This may simply be Lanfear/Cyndane's 'end game' strategy now that it is clear that she failed to convert Rand.

Also, at the Cleansing, it is clear to me that Cyndane was commenting that Alivia was stronger than Lanfear was. This is somewhat different than saying she truly experienced a power loss, because it implies that she truly is trying to forge a new identity...and as far as anyone else knew, the woman standing in front of her would have been stronger than Lanfear, which is probably what ticked Cyndane off enough to weave enough Earth to trigger an earthquake before the fight began in earnest. This might imply either a willingness to create a new identity or actually a form of psychosis where Lanfear died, was Reborn, and then transmigrated so that, in the same way that Rand identified his past lives as being separate from him but his soul in different lifetimes, Cyndane knows she is Lanfear, but has lost some of the personality quirks, noteably pride and arrogance, that came with the Lanfear persona. Otherwise, she would have said "Alivia is stronger than I am!" rather than implying the period of time had something to do with what people would think of Lanfear.