I had something similar on my 4/4 it turned out to be the coil, I still do not understand it but the car had been running fine but after a wash it was really running rough, I had had that before and once everything drys out all is well, but not his time. Like you I set about checking everything, fuel, timing, spark, plug leads. Eventually I changed the coil for a new one I had and it instantly ran fine, presumably the old one was not producing a big enough spark.
