Lord Tim, OK, will do that today.  Thanks for all of your help, man.  Also, concerning M-Audio, I called the other day when I thought it might be the keyboard.  Very surprised. 
 
They're based in RI and I live in RI, too.  So, it was a local number.  Cool!  Just a side note.  I explained the problem, the tech guy sounded like a sixteen year old kid that had no idea what he was doing.  It literally sounded like was sitting in front of a computer and, whatever problem I told him, he looked it up and read a scripted response.  It was pathetic.  Very surprised by M-Audio in that regard. 

I also have had this problem! Using a novation 61 impulse, it's been excellent until Sonar (im using professional lexington).
When i reopen a session file (all midi by the way), there's always a significant drop in tuning.
Previously, i would flick the pitch wheel and everything would snap right back in tune, though there is no record of it in the midi lane. You are NOT alone. it is simply maddening.
how can one immerse herself in synth-fu heaven if the midi ain't sticking right, y'all?
tears.

I had this exact problem as well as the lower octaves, say below middle C coming and going. Turned out to be a dirty slider on my Roland keyboard controller. I found it by watching MIDI messages and saw tons of weird ones going by. A little De-Oxit and all has been well since.

Yep, below middle C is where all the problems happen. oddly enough it works fine with my other programs.
gonna patiently attack the midi mapping from scratch like i should have in the first place.
used a "ready made" ACT profile(i think that's what it's called), but "i know i should know better."
sidebar:
i really love the hackability (versatility was almost the word i was looking for) that sonar presents. 
gets my wheels turning!
but, in the meanwhile..mixing is waaay more fun to do...
 

I never really had much issues with my M-Audio interface except that Windows drops/loses it occasionally and I have to reload the drivers.
 
I can confirm issues with different USB ports though. Slightly off topic but I have an external USB hub and use a portable external HDD. In some ports of the hub, the drive just will not work whereas other devices work fine. So it may be a USB port/device combination issue.
 
On a similar note, perhaps a particular piano sound is causing the issue? I ran across an instrument the other day that was about a full step out of tune. Change to another instrument and all was ok again. I wish I had made note of that instrument.

Hey everyone, just another update:  everything seems to be fine now.  This morning, I checked a test project I had recorded last night in the key of G minor, just a little thirty second thing with electric guitar and Rapture on another track.  Booted, rebooted, saved, rebooted, closed Sonar, rebooted, blah blah a hundred times, never changed key.  So, at least for the time being, my problem seems to have gone away. 
 
You guys really are awesome, though.  Happy New Year to all of you.  Oh, another side note:  I e-mailed Avid about my C-600.  I asked if they perhaps were developing a Windows 10 driver for the C-600.  Long shot, but it doesn't hurt to try.  The reply I got:  "Unfortunately, Pro-Tools is not compatible with Windows 10."  WTF?????

Hey everyone, one more question.  I figured I'd ask it here rather than make an entirely new thread because it's somewhat related. 
 
First of all, the pitch problems have gone away.  YAY!  Here's a new problem, though.  My interface is not getting power when turning on my computer.  I have to unplug the USB, then plug it back in and it's fine.  Any ideas?

Nevermind..... for those interested....
 
DEVICE MANAGER - USB - RIGHT CLICK ROOT HUB - PROPERTIES - POWER MANAGEMENT - DESELECT "ALLOW COMPUTER TO TURN OFF THIS PORT TO SAVE POWER"

DrLumen
Slightly off topic but I have an external USB hub and use a portable external HDD. In some ports of the hub, the drive just will not work whereas other devices work fine. So it may be a USB port/device combination issue.

I would use a hub ONLY for devices like keyboards, mice, dongles, printers, and other low-bandwidth devices.