CPU Usage Suddenly Started Spiking Mysteriously

I’ve been working on a project for nearly a month and the CPU usage has been around 35%. I closed the file one night and reopened it the next morning and didn’t change anything on my system or the project but now suddenly the CPU is up near 100% and the audio is just stuttering. I’ve rebooted and run extensive virus scan. I checked performance monitor and it is only Cubase that is running the CPU so high. I checked the driver is still the correct one and the buffer hasn’t changed. I then disabled all the plugins and the CPU drops and the file will play. I’m confused why with nothing being changed by me the same project I’ve been working on is suddenly crapping out the CPU? I never added any extra plugins or VSTs but suddenly this CPU overload occurred. I tried running the system when it’s cold i.e. first thing in the morning with the same overload result so it doesn’t appear temperature related. I also opened an earlier version of the project and it too is overloading as are any others that I previously could work fine with. Any kind suggestions please?
Thank you.

I’m very happy and relieved to report that I found the source of the problem. I am using a laptop and the power supply plug was fractionally not fully inserted. When I pushed it all the way in the full CPU juice came back!
Quite interesting as there was no indication I was running on battery power. Maybe there is an in between state between mains and power on Dell laptops?