W reg UK subaru. Has a miss ( pulling back) at 60mph then goes then again pulls back. changed plugs, leads, new belt, timing done, injectors etc.. mechanic cant find any reason for this...help? Diagnostics shows no problem with ECU, etc

Hi Gerard, on a 1.28v Punto the plugs are on the back of the cylinder head tucked between the 4 inlet manifolds. The plug closest to the timing belt end (just next to the oil dipstick) is easy to find, the others are a bit obscured by the plastic housing that sheaths the injector wiring. Be careful pulling off the suppressor caps, they hang on quite tight so best use long nose pliers and NOT pull on the HT lead. Hope that helps. Andy

Have a compression test done on the engine. Check for burnt valve or bent valve on that cylinder.

As long as the oil level is still good then, no, the oil can never affect and engine this way. The flashing check engine light means you have an active misfire. Which is most commonly caused by over due spark plugs or a shorted plug, old or bad plug wires, a bad ignition coil, a bad injector. If you pull the plug wires off of the coil and it still has that shiny gold/brass color then the coil is most likely ok, but if it has a bit of light greenish power on those same surfaces then you need a new coil. Also, if you do have the 2.5L single overhead cam engine, then make sure you know if your timing belt was done at 105,000 miles. Because if the timing is off due to an old belt, then that might be causing it as well. If you suspect a bad injector, then I would run an injector flush on it or just some fuel injector cleaner additive through your fuel tank and hopefully it will clean out the injectors.
I hope this info helps!~subyguru~

yes, there is a big belt and it is a job to replace, remove just about everything up front, radiator, timing covers, fan, belts, pulleys, hoses, etc, I think you change the water pump at the same time, not sure, But get the new belt with timing marks on it, they come in real handy, clamp the cams so they don't retract, before pulling the belt, put a pin in the belt tensioner hole if it has one, then pull the belt off, align all the marks, they have to be perfect, or don't bother. When it is all set to the marks, release the tension on the tensioner and spin the engine by hand with spark plugs removed , a couple times, when it comes around to one mark again, check them all, they all should be right on the mark. If you can't find a belt with the marks on it already, mark the old belt, with a white out pen or junk yard pen, relay the marks to the new belt. Then try it, it is most important that they line up. Crank and cams, all have marks on them. Hope this helps.

check maf sensor, cam/crank sensor, map sensor, fuel pressure, clean injectors, engine timing

Yes it could but i would suggest checking the crank position sensor as it will not throw check engine codes. Also go to an auto store and have them pull the codes for you for free.

If one cylinder is missing at idle, it would be useful to know which one. Pulling the plug wires one at a time with insulated pliers would tell us if one does not cause a change.
It could be a fouled plug.
You could switch plugs with another, to verify.

If not spark, could be fuel.
If throttle body, then not likely.
If individual injectors, then pull whole rail off and see if all injectors fire, spray pattern like cone, and none dribbling.

If none of above, then consider intake leak, collapsed lifter, valve adj, etc.

Subaru recommends replacement of the timming belt at 105,000 miles and a timing belt inspection every 30,000 miles.

either head gasket or valves burnt out ,either way pull head off to investigate

The cylinders are numbered as on the spark pack you changed. You might have done better putting the seafoam in the tank as an injector cleaner. Not sure why you changed so many parts, you seem to know how to get the codes from test connector lead hookup and the blink pattern. You might also want to check the fuel rail pressure and the crank and cam angle sensors. Before replacing the injector, swap the #4 injector with the adjacent one and see it the code moves with the injector or stays put. If the code stays on #4, then check the injector wiring, if it moves clean or replace the injector.