GM, I had not flown in awhile, but this morning I feel uneventful from Lisbon to Gatwick on a A320. That's the way it is some good, some not so good Dave has seen a couple of twitch video's I informed him of.
I can tell you at cruise it always looks the same, aircraft under control and then instantaneously it's at a different altitude and or speed. Warning alarms have been activated, there is never any advance indication, it just glitches.
Since flights occur properly and they are all flown under the same procedures, I can only rationalize this as a event that happens randomly and a glitch. I'm obviously not a developer on your system, but it seems like the code should be executing properly and some event occurs to make it not execute properly. When I worked we had the similar kinds of random events and had to basically put sniffers and other instrumentation on the device to trap the fault to analyze it. That was a few years back, technology has probably moved beyond where it was.