I have the same problem aswell !
I was doing a flight between LFPG and EDDF. Aerosoft A321 v18.104.22.168.
I added my flight plan in attached document. I was cruising at FL290. I didn't remember exactly when the problem started (maybe around FL240 ?). It also appened to me in another flight.
I'm currently descending to EDDF, 4.5 PSI
I'm using classics addons (ASP4, Rex sky force, T2G LFPG, Aerosoft EDDF, Orbx ...) and also FS2Crew voice control.
I will try to do another flight to give more info