Rocker bores worn oval, causes a "swinging" motion to the rocker making it impossible to get them adjusted properly. All it takes is a few moments of oil starvation or foreign material in the oil, like metal shavings or sludge to block the oil port. Yep, head comes off. Prepare yourswlf to replace the cam and rockers. Be sure to mikke the rocker shafts as well, they can wear enought to cause problems with new rockers.
Since you're there, replace the valve seals as well.
