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Hi and welcome to JA,
there is a re-calibration for the 96 fiesta but it may not cure the fault you have as it was more for low speed manouvering, (parking, turning round etc) the hunting/loosing power could be down to a number of things, hunting in general only occurs at idle, and on this engine 9 times out of 10 is down to the idle speed control valve either sticking or not working, you can try removing this and cleaning it, either with a carb cleaner aerosol, or by soaking it in petrol, DO NOT SCRAPE INSIDE THE VALVE! as it may not seal properly if damaged, it is also worthwhile checking that all vacuum, pipes and hoses are connected properly with no splits or leaks, and that the inlet manifold gasket is in good condition, the loss of power at high RPM could be down to the usual suspects, plugs, leads and coil pack, or possibly loss of fuel pressure due to a weak fuel pressure regulator, or fuel pump, a fuel pressure test and road test together could help determine that,
However, with regard to the oil loss, these engines, especially the early ones were very prone to excessive cylinder bore wear, this could be causing the power loss due to lack of compression, it is also very likely the reason you are losing oil, the oil goes past the pistons in the cylinder and is burnt off, I recommend you have a cylinder compression test and oil pressure test done, if cylinder bore wear is found unfortunately it is unrepairable, the cylinder bores in the zetec-se engine are nickel plated, once this has worn off the block is scrap, overboring is not an option unfortunately, and only a replacement bottom end will cure it,
hope this helps,
the revs are staying up because the vehicle speed sensor is telling the engine ECU that you are still doing 70mph, when you are 'in neutral' do you select neutral and release the clutch? or do you just hold the clutch pedal down? the clutch pedal has a position switch which informs the ECU when you are changing gear, these can cause a problem when faulty, it can also be attributed again to the idle valve not settling straight away,
with regard to not accelerating in 4th or 5th, this may be down to the oil loss problem, if the bores are worn you will have limited power, the engine has to work harder in higher gears to keep the car moving,
as stated in my first answer I really do recommend getting the 3 pressure tests done, if nothing else they will rule out fuel pressure, and any mechanical issue with the engine,