Bracpan;
Spending big for the 123Ign is a lot of outlay for what is not necessarily an Ign issue! I'd advise only considering spending that after you've confirmed Distrib was at fault...and you haven't yet anywhere near convinced me that "miss fire on full throttle on a long hill" is due to Distrib issue...have you checked Cent Advance, statically, and/or with a Timing Light? I'd recommend that first. (See:
https://www.sw-em.com/Volvo%20Igniti...e_Spark_Timing )
Without further proof or at least some substantiation that it's due to Ign, I would say misfire is just as likely a function of carberation...
My rule is: Proper and methodical troubleshooting before major outlays!
Good Hunting