What does it do if you just stop for a bit and open gas cap for a few minutes then put cap back on and start it up?????? do this and make sure engine is same temp and amount of time is the same-----

Sounds a little like maybe your tank/system isn't vented---faulty cap? is there a sucking sound when you loosen cap?

If its not from fuel system--it could be that the valve stem seals have failed and fallen off the stems----then they block the oil return holes, valve covers fill with oil, loose stems/guides-oil runs down into ports/cylinders fouling plugs on start up after SHORT interval--------if its allowed to set for hours and cool completely off the oil will seep back into the pan---------