Splat loads okay but when I try and load a project nothing happens it's cpu usage drops to zero and goes into not responding?
Anyone come across this?

What about loading new projects (with no VSTs) ?
Any difference ?
 
You can try loading your established projects in safe mode by holding down SHIFT while opening a project.
This will eliminate the possibility of a VST causing Sonar to not load projects.

In a way I have answered my own question, I changed the core management profile from balanced to highest performance and it seems to have done the trick at least this time. But the export to wav file chewed up 25% of 10 cores in cpu.

The strange thing is I opened the same project the day before to record a guide vocal track and also turned off some of the plugins. Also it opens with no issues on back-up PC that only has 6 cores with no issue and the vst's are in step with each PC currently.
I can load new projects okay which is just as well as I have a video call this afternoon to go through the process of recording the lead vocals on Sonar Artist with another member of the band against the mix I have just created.
The other possibility is that some of the pre-sets have not been authorised on the new PC but I thought I had covered all those that were affected by motherboard change.
Just in case I'll check the registry threads it's loading and any inconsistencies in versions in the paths, had already found one with Softube Saturation knob that is a lower version in Sonar compared to Softube but they usually get flagged by a CLSID conflict.

You would definitely help yourself by listing your complete specs. that includes Soundcard and everything.
I seem to remember you having a similar issue a month or 2 ago with Sonar freezing ? is that correct ? 

I have listed the complete specs the sound card is Sound Blaster ZxR Pro, yes did have same issue and now on a upgraded cpu replacement motherboard after a week of proving to Dell that other programs like excel showed similar symptoms.
But it is no longer a cpu issue as all cores have spare capacity and in any case SPLAT was using 0% cpu when loading project and I/O should not be an issue as on SSD.
But changing the profile to high performance as mentioned seems to have cured the issue as by default it was trying to balance the load which may have caused SPLAT an issue.
Will be doing further tweaking if required and once I am satisfied will publish results. Just ran a game and as expected profile changed to bitsum highest performance and no issues. What is puzzling is that backup-pc which has only 6 cores, same sound card etc no issues at all?