This is going to be quite difficult to explain + I’ve raised a formal support case with Steinberg but I am swinging it by you guys in case anyone has some ideas/suggestions
Here goes - this is quiet a long post 
I have an early 2011 MBP with 8GB RAM & a OCZ Vertex3 SSD plus a Prop. heads Balance Audio interface. I run Cubase 6.5.3 a virus TI polar, Nexus2, Omnisphere, Ozone5 ETC and I am working on a project that has 4 midi tracks and about 5 audio at the moment with various EQ/FX and a side chain ETC - so nothing too complicated.
Up until the weekend, cubase has been rock solid and I have had no issues with it or the track I am working on or my laptop. However, on Sunday my battery properly packed up [MAC would just keep rebooting when trying to load the desktop - it would boot into single user though] but as soon as the power was removed the laptop would instantly turn off - So yesterday I had the battery replaced at my local MAC store and the laptop is all working again and I am £100 lighter in my wallet 
I went to work on my project last night and noticed that cubase was a lot slower at loading the project that it has been in the past, normally its pretty snappy [due to the fast SSD] - the project eventually loaded OK though. However, as soon as I try and play the project the CPU load goes through the roof and every single bit of audio is corrupted/distorted, If I watch the timers in the transport bar they are all over the place and they do not keep tick time properly [4/4] and the ASIO meter is maxed out. Eventually, all audio is lost and looking at the process table - kernel task is the top usage and cubase is using like 70% CPU.
So I quit the program, reboot the system ensure that nothing else is running and try it again - and exactly the same happens. Also after one reboot I left the laptop for say 30 secs before powering it on and loaded up my project in Cubase again. As soon as the tracks started playing everyting was perfect - few I thought … there was no distortion and the timeing was kept, this lasted about 6 seconds and then the audio started to slowly distort and got worse & worse and eventually all audio was lost and I had to quit the program as it went almost totally un-responsive.
I’ve attempted to move my balance USB drive [which is currenty connected in a powered hub] but this made no difference. I even tried to run the project without the Virus TI. It’s still the same - everytime cubase is loaded with my project - it totally corrupts all the midi/audio output and even the mixer level lights become sluggish -and the transport timers are all over the place - the system is obviously becoming CPU bound somehow.
If I quit cubase and say load Reason 6.5 stand alone and work on a seperate Reason arrangement - the audio is rock solid - the prog is fully responsive and the system is also just fine watching movies or playing tracks through iTunes ETC.
The issue definatley is isolated to Cubase. I’ve also tried adjusting buffer sizes and cpu boost settings but this makes no difference at all. I do use a 32bit plugin via the bridge which was working just fine before so I tried forcing cubase into 32bit mode and I even disabled the plugin / removed it from the project and restarted cubase in both 32 and 64 bit, but exactly the same happens.
I really need some help here to try and diagnose the problem - I didn’t get time last night 6to test cubase with a fresh project but will tonight - this is bar far the oddest problem with a computer I have ever seen 
Any suggestions/commnets/points/tests ETC gratefully appreciated.
Thanks for reading this far! Kev.