If you have not synced the Tyros and Sonar in any way with midi, it is possible that their clocks are slightly different. Initially I thought you said there was a dramatic tempo change, but if you're saying they just "drift out of sync" over time this could be a normal clock difference. Meaning, for example, the Tyros 120bpm is really 120.02 bpm.

Sorry, everyone, but this might be worth mentioning...
As you know, in the Track View, you get a "picture" of what you have recorded. I have examined this for the recording I just made, and right up until 58 seconds, the "picture" is in line with 'bar counter' (?) immediately above it? But by 1 minute, there is an obvious discrepancy: i.e. the recording has fallen behind the bar counter. All in two seconds. WTH???
 

Hi Sanderxpander. Are you in a position to direct me to where I can learn how to put this right? (Although I am puzzled as to why this has only now started happening after 10 years.)
Mark

What you describe is impossible.
 
Are you listening to the metronom whilst you are playing?
When you listen back to metronome and your playing, are you saying when you hit a chord they are suddenly no longer in time?

It happened. But probably coincidence.
Okay: just tried yet again.
Used a nice Latin rhythm at 120bpm, well above its ideal. I recorded it with the audio metronome playing. And everything sounded fine until 2min 4secs when I swear I could hear the audio and metronome lose sync. I have listened back and, sure enough, this is the case. Perhaps tellingly (you might know much better than me) there is ALSO a clearly audible 'blip' at the point where the tempo changes. I did not hear it while recording. But it is there for all to hear on playback, every time. A sudden dull sound, then the sync goes.
I know it all sounds ridiculous. But believe me, it is happening and I am at my wits' end.
Mark

Wait: Are YOU playing in time to the metronome or the auto accomp. in the Tyros?
Sonar and your Tyros rhythm section both have built-in clocks. Like two watches. They will drift apart.
 
But I still don't understand when the sync is being lost. You say it is fine on record, but not fine on playback?
 
I also don't understand how you are achieving the sync in the first place.
 
 

Basically, you can watch the sound being recorded. Just hit a single note in time to the metronome, your playing will produce clearly visible audio peaks. Do they line up with the grid as you record?
And are they suddenly somewhere no longer lined up with the grid when you have stopped recording?
 
 

"Are YOU playing in time to the metronome or the auto accomp. in the Tyros?"
BOTH. I PRESS PLAY IN X1 AND THEN START THE AUTO ACCOMP ON TYROS AS CLOSE TO THE SONAR METRONOME AS I CAN. I DON'T NORMALLY DO THIS AS, PREVIOUSLY, I HAVE ONLY EXPERIENCED A PROBLEM MATCHING THE TEMPOS WITH ONE PARTICULAR TYROS STYLE. (NOW, OF COURSE, IT IS HAPPENING WITH ALL THE STYLES)
"You say it is fine on record, but not fine on playback?"
No, I heard it go out of sync while recording. It was only on playback that I hear this strange blip, just before sync is lost.
"I also don't understand how you are achieving the sync in the first place."
Good question. Once recording is over, I remove silence from the track, then line it up so that it starts perfectly in time. This has ALWAYS worked perfectly, until this week. 
Mark

"Basically, you can watch the sound being recorded. Just hit a single note in time to the metronome, your playing will produce clearly visible audio peaks. Do they line up with the grid as you record?
And are they suddenly somewhere no longer lined up with the grid when you have stopped recording?"
Okay, gonna do that now.

"Do they line up with the grid as you record?
And are they suddenly somewhere no longer lined up with the grid when you have stopped recording?"
Interesting. I would say that, with this experiment, the loss of sync was gradual, not sudden. I could both see this (by watching the peaks as you suggested) and hear it. It stared off in sync, and slowly but surely the tempos were different. What do you reckon?
Mar