Lothar,
Gas bubbles in the radiator are definitly not a good sign. Whether it is a cracked head or a bad head gasket is hard to tell. Either way, you don't want to drive this. I would suggest towing it. A cracked head or bad gasket is fixable. If you drive this it could cause a much bigger problem.
TimK
85\' GMC Sub, 4x4, 3/4Ton, 6.2L Diesel<br />67\' Land Rover Series 2A, 2.5L Diesel<br />00\' VW Beetle, 1.9L Diesel TDI <br />90\' Range Rover - soon to be Diesel<p>So what do you do with gelled Diesel fuel? Smear it on toast and have another cup of coffee until it gets warmer outside.