Hello everyone. Further to a recent post of mine, I can now describe the perplexing issue with a little more clarity...
When recording audio via a Yamaha keyboard,  the tempo within X1 suddenly slows down. At first I thought the keyboard was being erratic, but, no, it is definitely the software. So, for example, if I am recording an auto backing instrument at 120bpm (with the tempo correctly set within X1), all can be fine for one minute, or for 5 seconds... but sooner rather than later, the two bpms suddenly diverge. Originally I only realised upon playback. Now,by using X1's audio metronome, I can clearly idenitfy when the tempo of the keyboard and that of the metronome part company. Sometimes this change is accompanied by a short 'buzz' sound. For me, this is a major problem. It is as if something suddenly upsets X1 and causes it to 'hesitate', throwing everything out of sync. I have checked the Yamaha with other metronomes, and it is keeping perfect time.
I have ensured that my Edirol soundcard has the right settings, plus I have tried everything I can think of to locate the source of the issue within the X1's settings. Nothing. I have tried closing the Internet connection... no joy. And I've reinstalled X1, with no improvement. Something is interfering. Do any of you kind people have any suggestions? 
 

Hi. Are you sure that you do not have any tempo maps (hidden or not), that changes your tempo? Is you keyboard connected via MIDI, Audio or Digital (Spdif or the likes)? Could there be a mismatch (bit / depth) between your keyboard and SONAR if connected digitally? Does this happen on one or more projects?
Best of luck.

Are you recording something that you "recorded" in your yamaha keyboard?
If so, remember that either your board (or Sonar) needs to follow the other. 
One will be set to internal.  The other should be set to follow either by MIDI sync or MIDI Timecode

"Sometimes this change is accompanied by a short 'buzz' sound."
 
This, to me, indicates your system is getting slammed and throwing things out of whack.
 
Post your system specifications (including all hardware, driver mode, anything you can provide). In the meantime maybe look at your MIDI buffers (in Preferences). Maybe they can be tweaked to help the issue.
 
Also, as Arlen pointed out, you need to make sure your devices are properly sync'd. That is covered in the Reference Guide (and you'll probably want to look at your synth's manual too). I'd imagine devices going out of sync with the DAW could cause buzzing as well... but definitely timeshifts. It would be weird if it was that drastic though (I'd expect a slow drift over time) so I do think your system is choking on itself.
 
If all else fails give support a call.

Thank you so much. To get me started (and to indicate how little I know about the software)... how/where do I locate a tempo map?

Go to Views > Tempo. That will open the Tempo view in the Multi Dock. This will show any tempo variations (if there are any).
 
However if there are tempo changes they would appear in the Transport Module (in the Control Bar) during playback as the changes occur. So where it shows tempo, let's say 120bpm, if there is a change in tempo, it will change to another entry like say 110bpm or whatever.
 
Basically whatever goes on in the Tempo Map will be reflected in the tempo window in the Transport Module. If it stays constant throughout the track there are no tempo changes programmed in the project.

To elaborate ever so slightly on the excellent advice given by Beepster, when you open the Tempo View, there will always be at least one tempo event - hopefully the one you specified at the beginning.
 
This will show as a completely flat line running horizontally across the page, and you can also verify it by looking at the tempo list on the right of the screen. If this list isn't visible, hit ctrl + t to display it