Re: eb v8 3 probs in one....help!!!!!
Not being a panelbeater I am not 100% on what the problem might be here. I am only a mechanic, but will give it a go anyway.
The vacuum tank behind the guard is a storage tank for the heater controls only. There is a oneway valve fitted to the heater system which stops vacuum "bleeding back" when you put your foot down. These little valves can stop working, or you can develop a slight leak ANYWHERE in the heater circuit that will cause the heater to play up when you accelerate. A small heater vacuum problem won't affect the way the car performs.
An air lock in the gas convertor will cause the convertor to ice up very quickly. It is practically impossible for you to drive 400 kms and then have the convertor ice up. This will happen if you have a slow coolant leak, and it takes 400km for the level to drop down sufficiently. Plus, the engine cooling system on all fords from EA onwards have a pressurised header tank, plus air bleeds and a draw down hose to ensure that the engine does not have any air locks. It is reasonable to say that this would "extend" to the convertor hoses on this vehicle and eliminate the chance of an airlock.
Unfortunately, as you have a 5L, the heater tap plumbing is a little more complicated than normal. On a standard Ford there are only 2 hoses, and you can actually do away with the heater tap (you don't need it - FACT), and therefore ensure a continuous flow of water through the gas convertor. I have done this previously, and everything works fine, as Ford's have a Hot/cold flap in the heater box. But, you will have 4 hoses on the heater tap, and I have not personally had experience with plumbing a convertor into this setup.
Now, after all that...... I would suggest you have someone check the engine for a MAJOR intake vacuum leak. This will cause the engine to run lean, and give you your symtoms.