My au wagon s3 has most of the back end of the exhaust gone by rusting away, i found a few loose end plug leads which only got the running of it better, basically when i accelerate it missfires on and off, it also does it mainly up hills, the pulley main belt is fine with no teeth missing, it idles fine with an exhaust gasket sound leak. Would it be that the rotten exhaust has put out the timing. Holes in it start from the back of the cat. around the muffler then theres nothing at al there pretty much to the tail....

Check the resistance of your leads, they could be failing under load. This goes the same with coils.

Have you had a look at your fuel system? The fuel filter might be heavily blocked causing less fuel to be delivered under load.