I see some weird cyclic behavior too, not exactly the same but like this. If Sen is overloaded and walking back to Hlint with some stuff, there will be a period of time when all the sudden he goes on overdrive for a bit and after that he goes back to the right (slow) speed. And I always notice some weird periodic cycling of that speed indicator in the corner, it just doesnt always have a visible effect on the character's speed, I notice the effect when he's supposed to be slowed.
Maybe there's some effect that periodically turns off speed modifications for a while - making Sen go faster if he should be slowed and the barbarian go slower if he should be accelerated?