Maybe try this to determin if you have bad copper sealing rings inside  the
head. Use a common household carbon monoxide detector($20). Remove the
radiator cap. Get a 1 gallon milk jug and cut the bottom 3/4 of the way
off.Duct tape this jug to the opening of the expansion tank.Put the
detector inside and tape the bottom closed. Plug the detector in,and start
the engine and run it for the ammount of time it takes for your problem to
show itself.See if CO is in the coolant. Next , remove one spark plug at a
time and  and ground it ,to repeat the above proceedure. This will tell you
which head has the problem.Good luck.