The engine is basically a compressor , the faster you run it the more pressure will build in the crank case . So at wide open throttle if there is a problem there becomes more crank case pressure than the PCV or vents can dissipate. So what is causing the crank case pressure to build up to push the valley cover inside out. Broken rings, hole in the piston, wiped exhaust lobes all would be likely candidates. It might seem to you that it's running OK but your engine has a problem that stuffing in a rag is not the proper fix.