Tuning Instability when Restringing Low E

Restringing a mate’s Martin Smith and for some reason the low E is stubbornly out of tune. As I tune it up the peg becomes significantly resistant and eventually just kind of wavers between C and D, sometimes falling all the way down to F.

Have I simply given it too much slack (I allowed it the distance of a tuning post)? The bridge saddle also simply rectangular without any more shape, which I haven’t seen before.