It does sound like you are retarded (not you, the car! lol) or very lean! The spluttering and loss of power point more towards fuelling whereas retarded timing will just make the car down on power but still drive pretty smoothly.
As Jano says either try and get a known good metering unit or save for your management conversion, I personally wouldnt go Cossie as the ECU's/looms are old and been in and out of any number of cars, IME you often end up spending time trying to fault find broken wires and poor connections... There's some very good standalone options avaialbe these days brand new that cost a lot less and come with many more features too!