Simon
If the problem got worse right after you swapped the cap and arm, did
you get the firing order right when you reconnected the HT leads, also
are all the leads pushed right home in the cap and coil?
A failing coil or condenser can give the symptom you describe, also the
dwell angle and timing needs to be spot on.
Check for distributor shaft wear (sideways force), check the mechanical
advance is working.
You can check both these last things with a strobe timing light.
Also, you can trace the misfire using a timing light if the flex is long
enough for a friend to hold it and watch in the passenger seat while you
drive the car and provoke a misfire. You can monitor the coil, (see if
its the coil or downstream) or the plug leads individually this way.
I went round the loop with a misfire upon acceleration for a long time
before I realised I had fitted the wrong spark plugs, which were too
"cold" for my engine.
Also, its not unknown for new parts to be faulty.
Matt
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