Just replaced my old PC with a new Windows 8 64-bit unit, upgraded to Win 8.1 - I installed Sonar 7 Studio software from the source CD, then installed my X1 Producer upgrade, together with the X1 a-d patches - all set to 64-bit when given the option. After a bit of fiddling around with the drivers to get Sonar to recognise midi - by setting audio to MME 32-bit - I can now open projects up without getting midi error messages.
 
The projects played fine on the old PC running Vista 32-bit. The files were transferred to the new PC using Windows Easy File Transfer.
 
Anyway I've now struck a problem - I use the session drummer soft synth for the drum tracks, created using the step sequencer. On playback the audio tracks recorded on the old PC are fine, but the previously created drum tracks now only play random beats out of the sequences - it's not consistently NOT playing particular drums, they're all there, but it's almost like the volume is constantly fading in and out.
 
Any ideas ?????

I think I may have solved the problem by changing the audio->driver settings->Buffer Size to "Fast" ??
 
However, I'd be happy to hear any other suggestions

I was going to say it sounds like your midi settings were reset upon new install of sonar.
go through all your preference menu settings to make sure they are all in check.
this will avoid a lot of headaches like this one here.
 
good luck