Hi Tim, i can confirm what John says - check the exhaust water injection bend where the water injection point is. Remove the whole thing (just undo the exhaust and water injection rubber hoses and the bolts from the head flange)
Check that the inner pipe is not perforated as this lets injected cooling water go back to the exhaust port at the head. Also check the head exhaust port from the manifold flange back to the valve is at least as big as your index finger all the way through . This is where a build up of crud (salt and rust?) can build up and choke the engine. If this crud is excessive and old, it can also lead to perforation between the exhaust port and the coolant chambers which surround the port.
I had to replace the head this year when I had the same problem.
Despite the cost of new exhaust injection bends, seems they may be cheaper than new heads!
Best of luck
chrisN