Engine overheating

[Mcgi7]

Quite a lot of info here so I'll try and make it as coherent as possible.

For the last two months my mk7 Fiesta has been telling me it's overheating on the dash. On a typical drive it sits at 4 bars (standard operating temp) until I hit an incline where it will increase gradually until the warning light comes on if the hill is steep/long enough. In addition to this, towards the end of a drive it will begin to increase to 5 or 6 bars when I've started to slow down and park. When I stop the car you can hear and see aggressive bubbling from the pipe back in to the coolant reservoir. After a drive, even if the car has been sitting idle for up to 2 hours, when I restart the car it will be sitting at anywhere between 2 and 4 bars. 

My regular mechanic took it in to look at twice within the first week of this happening, at first disbelieving me that there was even a problem and then coming to the conclusion that it was software related as he was unable to find any part of the engine overheating, the fan was coming off and on at the correct temperatures and no fault codes being shown by the computer.

I took it to Ford to get their take on it and they told me very quickly that the problem was due to the thermostats sticking which they subsequently replaced. This didn't solve the problem, however, and led to them using donor parts to test the radiator, water pump and engine temperature sensor as well as checking the software and performing a sniff test all to no avail. Ford then informed me that they thought it could be due to the leak in the head gasket but said it would cost me a lot of money to find out one way or the other, so I took the car back on the advice that it sounded like they didn't have a clue what the problem was and were gonna rack up a huge bill systematically working their way through the engine.

Has anyone ever had an issue like this/does anyone have any suggestions? I thought out of anyone Ford would be able to diagnose and fix the problem and I'm at a bit of a loss as to what to do. My regular mechanic has offered to have a look and I'm asking around the area for recommendations for other mechanics to take it to but I feel like I'm gonna end up paying a lot of money to get no further forward.

 

Any advice would be much appreciated

[tomshepp]

The coolant is boiling but the components are all checked by substitution. It gets worse under load but otherwise works ok. The coolant can be heard bubbling when the engine is stopped. 

RADIATOR CAP!