Most of my driving is around ordinary streets and roads at up to 80kph so I generally put it in 3rd not 4th because it sounds a little bit grumbly otherwise. Today I turned out onto the highway and actually had 4th selected and got to 80kph and it didn't shift up like it should have. Let my foot up and I'm pretty sure the revs dropped like it was freewheeling (not super sure) and it still didn't shift up, just stayed in 3rd. Couple of k's later I stopped for a few minutes and went in to see someone, then got back in the car and it behaved as it should. Tranny doesn't get serviced a lot but 12 months ago it got 3 fluid changes in 3 weeks to flush it all out and have only done about 10,000km since then. Seems to be in pretty good shape, never misbehaves otherwise, has a shift kit and Corvette servo and probably other stuff by the previous owner of the tranny. What might be happening? Is it possible that seeing 4th doesn't get used much that someting might be sticking?