VT-x not always enabled... any idea's?

Hi folks, first post and a bit of a conundrum for me.
On odd occations VT-x won’t enable on my desktop and I can’t figure out why apart from age of the machine.

Specs:
Asus maximums III formula / bios updated
Intel I5 750
8 gig DDR3
The usual hard drives / graphics
Running Win10

The issues it’s causing is when I go to run virtualbox. If VT-x is active it runs fine but when it’s not obviously vbox won’t run.

I’ve looked at conflicts, driver issues (grabbing at straws with drivers I know) and even stripped and rebuilt the system (it actually needed it… house dust city)

The only other thing I can think of is the age of the system as it’s now 9 years old and may not be as good as bit once was. And if I remember right the I5 was one of the first chips to have VT-x so maybe it’s not that great on that chip. Iirc, the earlier versions of the I5 didn’t have virtualization on them or it was turned off.

Any help appreciated

How often does it happen?