Ok mate heres some things to check and
how to check them.
Im thinking the ignition system, with the exception of coil and dizzy cap should be okay, as any fault in other parts should stay no matter what revs.
The coil is just one wire 'coiled' with plastic casing as an insulator, if thats dead it should usually be spottable. A coil going bad is usually moisture getting in, through even tiny cracks, or losing voltage 'arcing' to car body.
Hence on the coil you should look for scoring, white marks, pitted plastic or any cracks, if none of that move on.
The dizzy cap - pop it off, squeeze it lightly and check for any cracks in the plastic, on the inside there are four metal contacts which should be clean, sometimes they get white powder on them. More importantly is the carbon rod, look at the centre on the inside, its like a black pencil rubber, it should be intact and smooth.
Air filter have a look should be clean, then check the thick ducting from the air filter to throttle body closely for cracks. this is true for every single vacuum hose you can see in the engine.
I would check all above, if the fault only occurs when the engine gets up to operating temperature, then is okay if you leave it twenty minutes or so and start it up no probs, then same again, then it suggests a G28 speed sensor, however this shows up on a fault reader usually.
I would do the basics, then think about buying a code reader, about 30 quid off a popular auction site.