UE5.4 Windows 11 shipping build sometimes struggling to hit target FPS on my PC

I have a AYANEO Air Pro (AMD Ryzen 7 5825U) which in theory is a good gaming PC.

I am building a 2D game and am testing performance by having it set to a fixed frame rate of 120.

The game runs absolutely fine solid @ 120 FPS all day long! Yipeee!

BUT! then some days, the same build (packaged) will struggle and hover around 90-113 FPS!

Sometimes it may recover back to 120 FPS or it just won’t.

Sometimes a reboot will bring it back, sometimes a reboot makes no difference.

What’s going on? Is it Windows or is it because my PC only has 3 GB VRAM (which I can’t change)?

If it is VRAM then why does it on some days run awesome?

I have no anti-virus installed (other than Windows Defender) - no other apps open - all settings maxed out for gaming etc.

I’ve done all requirements within UE config to get the most performance.