Cylinder head bolts are stuck
 

I am attempting to remove the cylinder head from my 1997 XJ6 X300 car and I am at the point where I am trying to loosen the main cylinder head bolts. I have a 1/2 inch drive breaker bar (18 - 20 inches long) and a 3ft long piece of pipe stuck over that. When I first started trying to turn the main head bolts each one started to turn for approximately a 1/8 turn and then got very tight. Now I they are VERY difficult to turn. I am worried about breaking them,

Has anyone else here run into this?

They are 'torque-to-yield' bolt and will feel 'springy' until they 'snap-loose'.
Once you hear/feel them POP, they will turn normally.

I have NEVER broken one removing, but I have snapped a few upon installing a used one (Jaguar claims re-use twice).

bob