993 Tip transmission

[Thommo66]

I have a '97 993 tiptronic which I've owned for a few years and never had any issues with the transmission.   Recently had a service at PCM and all good reports, no red flags.   Yesterday I was driving in auto/D mode for the first 5 min following an initial 30min trip, then parked for a couple of hours and all of a sudden it would not shift up out of 3rd gear and remained revving around 3000rpm while ~85km/hr.   I tried shifting it across to manual and manually shifting up and down but nothing changed.  Stuck in 3rd.   It did shift back down, when in normal drive mode again when I slowed down and found a place to pull over where I could safely park and turn the car off.   I started it back up again after a minute or so and everything seemed to go back to normal.  

Anyone had any similar experiences out of the blue in a tip 993 ?  Any ideas on what the root cause might be and how to remedy other than taking it back to PCM to be checked out ?  I have not refilled anything since it had a major service around a month ago.