Does your room have a wooden floor?
When you take your weight from the chair by getting up, a plank might reach where the PC sits, wobble it, which might cause the reset-button to be activated, because the reset-button is not properly connected.
Open your PC (after you switch it off first and disconnect it from the electricity).
There should be a pair of thin wires going from the front of that PC to the motherboard. The plug on the mobo probably says "Reset". Pull that off but remember where and how it was positioned.
Startup the PC again and move around, get up, sit down etc.
If it is still rebooting when you get up, you have a little Leprechaun hidden inside your PC.
If everything is hunky-dory, follow that reset-cable to the frontpanel. Either replace that switch with a "momentary" push-switch (from any electronics shop for a few cents/pennies) or check if there is a (unused) turbo-switch on the front. Use that switch for "Reset" and lead its cable to where the reset was on the mobo.
Let us know the result please, Leprechaun or wobbly switch.