Slow change from 3rd to 4th

[ronwest]

So this evening I'm boppin' along at about 80kph thinking that the engine was a bit louder than usual... window not fully closed?...no....what, oop's auto decided to change from 3rd to 4th.
I slowed down and tried to reproduce but it wouldn't do it on demand but at some random moment a little later it did the same thing.
I thought it did something (this) unusual once yesterday, as well.

Obviously, I'll check oil level but if it's more serious, what is it? Solenoid? Valve body? Electrical connector? Poor hygiene?