I was running Sonar Studio 8.5.0 until today.  For some reason when I try to import any supported audio format, the audio playback is "choppy".  However, when I run Sonar 2.1 on the same machine, the same piece of audio plays back just fine.  I am on an XP Machine and 8.5 has worked fine in the past.  I updated to 8.5.3 hoping that would solve the issue, but it hasn't.
 
Any other suggestions?

Check your audio settings in 8.5
 
Other than just saying "Ah, the good old days" that's all I can think of.
8.5 was very rock solid for us. I ran countless demos with real bands staring me down with it.

Which settings should I be looking at?

Buffer size. It's entirely possible that 8.5 needs larger buffers than 2.1 did.

I run Sonar 8.5.3 on a 32-bit Intel Atom-powered weakling netbook to record 28 tracks simultaneously using two Tascam 14-input audio interfaces, so you should get good results, too.

I brought the buffer up to 2048K and it still has the issue.

So these are only audio files?
 
I was never able to play the Cakewalk demos because the number of plugs just choked my PC.
 
But if it's straight audio - very strange.
 
Edit: does the cursor run smoothly or does it also jump?
Does it run smoothly on WDM? (latency is horrible, just as a check)

That brings up the notion that the keyboard pause key has been pressed. Press pause on the computer keyboard. Its a toggle. 2048 k is a too large for your setup. Too large a buffer is almost as bad as too small. 

 I updated to 8.5.3 hoping that would solve the issue, but it hasn't.     
 
Avast me harties....arrrr...
 
As a reformed Pirate myself I just can't help but advise the best solution is to upgrade. 
But if you really are stuck with an old XP computer then 8.5 was the best version for that system and it should work,, but are you using an audio interface? You won't get much to work without one.