HazelNuts39, I was about to shake my head "Yeah" to bearfoil's phugoid scenario when something dribbled into my thick skull regarding the timing of the transmissions. I don't know (or have forgotten) how long each ACARS message plus handshake takes AND what length of guard time must exist between messages. I also don't remember if BEA established AND published whether there had been any other transmissions that would have consumed some of the blank intervals.
Messages appear to take (an incredibly long) 6.5 seconds. There were just a whole lot of planes in the air that would be using that same satellite. Message frequency is normally very low. What is the usage rate of that particular satellite channel? Could some of those time intervals been consumed by other competing messages?
That aside some form of attitude instability would make sense with a periodic transmission interruption. However, the number of interruptions you have charted seems high to me.
I also believe that BEA did not comment about messages being delayed from the time it was expected the message would be sent other than the messages that should have been cued for transmission next after the last message ever transmitted.
That basically says, "This is an area that could be reviewed again." Once we know the duration of typical ACARS message transactions that can be added to your charts to show gaps that are suspicious. Then we'd need to ask if the message after a gap would have logically been sent earlier if it could be sent.Let's establish the phugoid spin as being near the same level of probability as other possible explanations.
I've long entertained a attitudinal upset or spin scenario to explain the gaps. Maybe it's time to establish how likely messages really WERE delayed. I've probably been remiss there.
{^_^}